How to Identify a Vintage Omega – Part 2

 In Collecting Watches

In the second part of this series, we’ll focus on another aspect of any vintage Omega – the case and its markings.

Over time, the patterns of markings have changed a few times. Some things should always be red flags, but some are legitimate oddities.

Location of the markings

The general rule is that Omegas have all the relevant markings on the inner side of the case back. These should include a reference or serial number, the Omega stamp in a triangular frame, and the case material. Sometimes, and that usually applies to 1950s and 1960s pieces, you can also find some info about the manufacturer of the case.

Case back Omega Speedmaster 145.022

This the case back of an Omega Speedmaster 145.022 from 1984. The CRS at the bottom means that the case was made by Spillmann.

There are some pieces that don’t follow this pattern. For example, the two collections made for the Swedish market – the Suveran, and the Officer. These had the markings on the outside of the case back.

Also, many US market models were cased locally, in cases made by American suppliers. These often had the case material marked on the outside.

Case serial numbers

While Omega’s reference numbers start somewhere in the 1930s, they were usually not placed on the watch itself until the mid-1940s. Notable exceptions include the Naiad and Medicus models, which did sometimes have the reference stamped on the case back.

The case serials follow the same code as the serial numbers found on movements. There shouldn’t be much of a gap between the case and movement serial – 2 years tops. Note that if it’s three, for example, that doesn’t disqualify the watch. Omega has a record of pairing movements and cases from batches made within a considerable period.

While the reference numbers started to appear on cases on a regular basis in the mid-1940s, the latest serial-stamped cases I’ve seen dated to 1947-1948. This especially applies to chronometers equipped with the 30T2 RG (all variants of the RG).

Case material codes

Every Omega reference was preceded by a two-letter case material code. This was almost never found on the case.

It was also – at times – not very consistent. For example, with 3-, 4- and 5-digit reference numbers, the standard code for stainless steel was CK. Still, there were models in stainless steel, where the code was MI.

For watches made until the early 1960s, the codes were as follows:

  • Steel: CK/MI
  • Solid gold: OT/OJ
  • Gold cap: KO
  • Gold-filled: PK

Remember that if a reference was made in a few case material versions, the base reference number will always be the same.

So, let’s assume that you’re checking a 4-digit reference in the Omega Vintage Database (OVD) on the Omega website. The only result is a watch in solid gold, while the one you’re looking at is stainless steel. This doesn’t mean that there’s anything wrong with the watch. It’s just that you’re looking at a CK, the steel version, and the watch that is listed in the OVD is an OT/OJ, in other words, a version in solid gold.

The OVD misses plenty of entries, both entire models as different versions of listed watches.

3-, 4-, and 5-digit reference numbers

It’s unknown if these reference numbers actually have any meaning. I don’t suppose they’re completely random, but there aren’t any means of decoding them.

The four-digit references were used until the early 1960s.

In the late 1950s, Omega introduced a short-lived 5-digit system, paired with the same material codes as the 4-digit system. It was largely phased out by 1962/1963.

The 6-digit system

In 1962, Omega introduced a new system of case reference numbers. By the 1970s, it was changed again, to a 7-digit system.

Case back Omega Seamaster De Ville

The case back/unishell case of an Omega Seamaster De Ville with the reference number 136.020. The CB at the bottom means that the case was made by Centrale Boites.

Still, the principle remains the same for both systems, that the first three digits define the characteristics of a watch. A key for this can be found here, along with case material codes for 6- and 7-digit reference numbers. So, if you look at the ordinary Speedmaster reference, 145.022, 145 means: (1)Gent’s watch (4) Manual winding chronograph (5) Water-resistant.

Keep in mind, that the OVD might require you to add a 0 after the period in a 6-digit reference to look it up. For example, a 145.022 becomes 145.0022 in the search.

American market references

While models with ordinary reference numbers were available in the US, Omega has made a number of watches for the US market only. Only a few of these are listed in the OVD.

Usually, the references of these models had the prefix “G.” If you come across one of these, chances of finding it in the OVD are slim. Again, this simply means that a watch isn’t listed there, which sadly isn’t uncommon.

Two references on one case?

That’s not unheard of. Certain cases were used for two different models. The difference between them could be a completely different dial and handset. Sometimes, though, it was the movement.

Omega DeVille Quartz case back

The case back from an Omega DeVille Quartz with two reference numbers.

Every now and then, one reference could have been used with two or three different movements. Apart from one reference spanning over two or three generations of a used movement, this is also the case with pieces made for the US market. The movement in these had a different number due to a different (lower) jewel count. They had fewer jewels to avoid a higher import tariff in the US.

A collection-specific number on an unsigned watch

What if the watch you’re looking at has a reference number that tells you it’s a Seamaster, but it doesn’t say Seamaster on the dial? That’s quite common. One reference could have belonged to two collections at the same time (for example, Seamaster and Geneve), and yet the watch was also available with no collection name on the dial.

A noteworthy example of a shared reference is the Seamaster 600, which was also available as a Geneve. But there, the Seamaster always had the hippocampus logo on the back. A plain back + Seamaster 600 dial = a “franken” or a redial.

Keep in mind, that a reference which was shared between different collections was a thing of the 1960s and 1970s. In the 1950s, a Seamaster reference on a watch with an unmarked dial only means that the watch was available without the collection name. Also, for a Seamaster made before ca.1958, the absence of the hippocampus logo is OK. That logo appeared on SM cases only from very late in 1957 on.

Basically, it’s the year of introduction that matters. For a reference introduced before 1957, the absence of the logo on a specimen from 1958 or 1959 is nothing strange.

Locally made cases

This only applies to watches that were made for the US and UK markets. Steel cases were always made in Switzerland, along with the rest of the watch. However, gold-plated and gold-filled ones for the US market were made by local suppliers, like Wadsworth.

In the UK, solid gold cases, especially the ones with the typical British purity of 9ct, were made locally by Dennison (ALD). A Dennison Omega case always has the following markings:

  • Omega stamp
  • Reference or serial number
  • Purity mark
  • Assay office stamp (London, Birmingham, Glasgow…)
  • A “date letter” corresponding to the year of manufacture of the case. Every assay office had a different letter assigned to a specific year.

The calibre doesn’t match the case

The first and most obvious reason for that is that the watch is a “franken”, which means that it has been assembled from random parts
However, there’s one exception.

Always check if the movement in the watch isn’t simply a US version of the movement that’s mentioned in the OVD. These will have a US import code (OXG) on the balance cock, and a lower jewel count. The OVD often doesn’t list the US version of a particular reference.

This is true for almost all families of Omega rotor automatics: 4XX, 50X, 55X, 56X, 59X, 71X, and 1XXX series. For example, the jewel count is the only difference between the 470 and 471, 490 and 491, 550 and 552. And so on.

You can find the details of the differences in the Ranfft archive. Every entry for movements within a particular family list the differences between the movements within that family.

Otherwise, as mentioned in the previous part of the series, a movement that doesn’t match the case is a red flag.


Keep in mind, that some vintage models are faked very often, especially the Constellation. These fakes will often check all the boxes for the marking patterns described above, but there are details that’ll tell you it’s a fake.

You can find a detailed guide to authenticate Constellation cases at Omega Constellation Collectors, which is the resource for Constellations.

All of this might seem very complicated. Still, just authenticate/identify some watches for the sport of it. As a challenge. After some time, it’ll sink in and become second nature.

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