Having looked in AJTT, all I can find is a listing of the calibre as:
"27.96 (or 27.9) NN S (3/0 size) 15p [no. of jewels] 1917 [year of first manufacture] also called 27.96x4.7 - rare".
In the section that pictures all the movements they only show one design, the one of the right of Yann's photo below:
https://omegaforums.net/attachments...5/?temp_hash=00ac204c7e4be4682c37484a0433f157
Otherwise, all i can find is a watch shown on page 109 that has the same dial as the OP's watch. There's a footnote that says "Launched in 1917 and also called the 27.96 x 4.7 mm, this calibre was only produced in a very small series." That's it.
There's no mention of anything about the regulator. Loking at the other movement designs from the period I can't see anything similar to this. There are some with what looks like more of a circular ratchet design, but nothing with a regulator shaped like this one.
EDIT - As pure speculation, I'd was going to make a wild guess the regulator part was perhaps added during servicing at some point - possibly because the correct part was rare or no longer available from Omega. However, looking at the OP's watch I don't think that can be right. The part is screwed into the plate by 2 screws on the top right hand side. The plate pictured in AJTT doesn't have such screw holes & neither do any of the similar movements from the period. So, if the possibilities are that it was a complex make do and mend repair or that the part is factory original, I'd favour the latter as the simpler explanation.
I'd love to be able to find out more if anyone else has one of these.
Click to expand...