I can't help with the case markings, all I know on that is the info Jeff posted on here from Hans - I don't have any early 70s Carreras with the case markings, one is a pre-improvement Cal 11, the others all Cal 12 after the change over.

I do have a theory on the silver-coloured Cal 12 movement with the I though. Part conjecture and part extrapolation, but it makes sense to me anyway! Let's start with a good rule of thumb - the Cal 11 movement is rhodium-plated (thus looks silver) and the Cal 12 is gold-plated (so looks...well, gold). We also believe that Heuer were exchanging parts when watches were returned for service, rather than junking the whole movement and replacing it with a new one (11 > 11 I > 12) - that makes sense given that it is relatively few pieces that did change.

But your watch has a rhodium base plate as well as two rhodium bridges, the main one stamped Cal 12. What's up with that?

Let's take a closer look at that bridge, along with a couple of other examples and see what we can see...

The first is an original Cal 11 bridge. Rhodium-plated as expected. The second an original Cal 12. Gold? Check. The third you will recognise as the bridge from your watch. See the difference?

The bridge on your watch has had a circle of metal removed and the 12 has been stamped in the small pit that remains.

Which is where the theory comes in. I reckon that, when Heuer got these in for service, they replaced the parts that were different, leaving what was effectively a Cal 12 in the watch but with the original rhodium base plate and bridges. After all, there was little point in replacing those as they weren't among the parts that has caused problems with the Cal 11. But to indicate that it was in fact the improved improved version, it looks as if in some cases they took the original bridge and redesignated the movement as a Cal 12.

Now, I don't know why they only seem to have done this in some instances. But no-one other than Heuer would really have reason to do it, so I don't think there are any issues with originality here. Perhaps some watches were restamped as part of a product recall measure, or just because the customer or distributor was particularly difficult and wanted proof that the issues had been remedied.

Maybe someone with Heuer contacts could ask if this was indeed what happened? Off the top of my head, when I have seen 12 stamped into a rhodium bridge, it was always in this little "pit" - anyone have any movements where that isn't the case?

Mark