Vintage Omega Seamaster with Dennison Caseback

It's a British version of the 2846 which was a late 50s snap on back auto using the 501 movement. Based on that pic, the Dennison case number was probably 4010 so that may be worth searching for info on.

EDIT cross posted with that above, as you can see we mostly agree but I would say that Dennisons do usually have a case number but it doesn't correspond with Omegas, I have had a 13308 which is a 501 movement model and also a 13322 which has the 30mm movement, both gold. Maybe only the gold cases had numbers.

A very good question. Perhaps Dennison were able to make the cases cheaper than Omega's Swiss case maker so it made sense. Post War, Omega didn't AFAIK make any cases in house it was always farmed out to CB, HF and countless others. Even Patek supplied Omega with cases. Dennison were a big concern making cases etc for many movement providers so perhaps its made some sense with economies of scale etc.