Help | 1910 Omega Pocket Chronograph - Identifying caliber and mainspring

António VC · Feb 25, 2026

Hello all!

Today I'd really appreciate your precious help… Here is a very special Omega Pocket Chronograph with a serial number that points to 1910.

It seems to be a cal. 18”CHRO but it has a chrome finish instead of gold. It also has a mark bellow the Omega inscription that I cannot clearly identify (an “M”, maybe?).

To the experts in this forum I beg for some help confirming the caliber and what mainspring should be the correct or compatible (similar to cal 33.3 CHRO or standard cal 18 - non CHRO?). I just can’t find any info anywhere that ensures these two doubts that I’m currently having…

gatorcpa · Feb 25, 2026

I think this dial is more or less the same:

Information on the caliber may be found here:
https://www.timeline.watch/watch/1918-omega-chrono-tachymeter-kca-141-lv/

gatorcpa

António VC · Feb 25, 2026

gatorcpa
I think this dial is more or less the same:

Information on the caliber may be found here:
https://www.timeline.watch/watch/1918-omega-chrono-tachymeter-kca-141-lv/

gatorcpa

@gatorcpa thanks for the reference. Actually I already came across that link and some other threads in OF. One with a similar example for sale.

I believe that’s an “M” next to the Omega inscription so apparently it’s from the French market. My doubts persist mainly due to the movement finishing, the resemblance with the cal.18 chro and the fact that it’s a split second cal. makes it different from the 18/20…

Tire-comedon · Mar 1, 2026

gatorcpa
I think this dial is more or less the same:

Information on the caliber may be found here:
https://www.timeline.watch/watch/1918-omega-chrono-tachymeter-kca-141-lv/

gatorcpa

Hi, this is a 19'' CHRO inside, not a 18''.

Tire-comedon · Mar 1, 2026

Hello,
Antonio's caliber is indeed a 18''CHRO, not a 19'' CHRO or 39 CHRO.
For comparison, here is a comparison between 19'' and 18'' from a 1926 spare parts catalog :

the three different calibers, all in their Chronotachymetre SGDG configuration :
19''CHRO (with only one subdial) :

18''CHRO (with two subdials) :

39'' CHRO (ref CK141 two subdials, smaller diameter) :

Now the chronotachymetre family is not complete without the 19'' CHRO duo-face :

group shot :

Now regarding the mainspring specs, here are the size for all calibers , from the same 1926 spare parts catalog (you will notice that the 18'' CHRO differs from the 18/20 non chonograph and from the 19''CHRO) :

I apologise for the long message, however I hope this may be useful.

JimInOz · Mar 1, 2026

Tire-comedon
I apologise for the long message, however I hope this may be useful.

No apologies necessary when it's an informative and entertaining post Yann.

It's also a slightly "jealousy triggering" post 😍.

PS: How do you get your background so black? Do you have a "secret light absorbing" material?

Tire-comedon · Mar 3, 2026

JimInOz
No apologies necessary when it's an informative and entertaining post Yann.

It's also a slightly "jealousy triggering" post 😍.

PS: How do you get your background so black? Do you have a "secret light absorbing" material?

for the black background I use a simple sheet of black Canson paper. Just remember to wipe out dust particles before taking pictures. The thing is not the background but the light. No direct unidirectional light source and you have something clean!

António VC · Mar 6, 2026

Hello @Tire-comedon ,

My apologies for the delay in responding but, honestly, I gave up thinking that nobody would help me more than I already have been enlightened by my searches. I must confess I am perplex and very, very grateful for your precious and extensive contribution! 😀 Lost for words...

I was pretty sure it couldn't be a 19CHRO but my doubts remained due to the caliber chrome finishing because the other 18CHROs that I've found were always golden. Every time I found a similar example, there was no reference to the caliber or when that was the case, I was pretty sure it was wrong.

You just solved my main fear: messing up with the main spring!

Once again, thank you so much! I'll post some pictures of both watches when they are both back to action. Just like @JimInOz said, I'm jealous too 😀

Tire-comedon · Mar 6, 2026

António VC
Hello @Tire-comedon ,

My apologies for the delay in responding but, honestly, I gave up thinking that nobody would help me more than I already have been enlightened by my searches. I must confess I am perplex and very, very grateful for your precious and extensive contribution! 😀 Lost for words...

I was pretty sure it couldn't be a 19CHRO but my doubts remained due to the caliber chrome finishing because the other 18CHROs that I've found were always golden. Every time I found a similar example, there was no reference to the caliber or when that was the case, I was pretty sure it was wrong.

You just solved my main fear: messing up with the main spring!

Once again, thank you so much! I'll post some pictures of both watches when they are both back to action. Just like @JimInOz said, I'm jealous too 😀

You’re welcome. No need to apologize, those watches have waited for more than a century so you can take one week to react.
If you plan to restore it, I may source a clean period non redone dial, just give me the exact diameter and I’ll check.

António VC · Mar 7, 2026

Thanks!

The one I posted is the worst and already went to a professional restorer. The other one is fine. Let me see the results and if needed I’ll get back to you, for sure!

Help | 1910 Omega Pocket Chronograph - Identifying caliber and mainspring

António VC

gatorcpa

António VC

Tire-comedon

Tire-comedon

JimInOz

Tire-comedon

António VC

Tire-comedon

António VC

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