The barrel Carreras have their reference numbers on the small lip above the lugs Timothy. Model number at the top, serial number at the bottom.

You'll probably need a very good loupe and good eyesight on top of that, this is some of the faintest engraving Heuer ever did.

And yes, Calibre 15 is correct, that's what the 150. in the model reference indicates - the Calibre 12 version is 110.573 (they never updated the references to reflect the Cal 11 to 12 change).