Mystery 1930s Omega 26.5 SOB with incorrect caseback

Whilst picking up a number of watches from storage recently I found this Omega which I don't recall and have no idea of it's history. It may have come as part of package of a deal and has sat in a box for 10-15 years at a guess.
I'm ok with certain Omegas but have no real idea on this one.
I assume the caseback is incorrect as the movement and dial would suggest it is from the 30's and I believe the movement is probably the 26.5 SOB.
I would be most appreciative for any thoughts about what model it is and if the dial, case and hands are all correct.
If anyone has (or knows where I could source) a correct caseback please let me know.
...and if anyone is in need of a caseback for a 2691-3 I'be happy to help out

Thanks in advance

Pls. Elaborate: screw in caseback and the case middle? With a thread or not? If so, it might be a fantasy combination. The movement screws are missing, not secured in the mvmt. Holder. Ex black dial with blued hands? Not very likely. Dial possibly changed. Rest might be correct. Or any other changed parts likely.....

The back is threaded as is the case and there is a retaining ring and a dust cover which may explain the missing movement screws.

A Dust Cover ? More pictures needed.

Looking at pictures of correct CK2691 watches, I would hazard a guess that the case/back go together, and the dial/movement have come from something else.



Maybe worth comparing the case to this NOS (although obviously yours will have some wear!)

https://urdelar.se/products/case-omega-ref-2691-1-cal-410