I did some digging on this for the OP, I spoke with some friends at the factory in Bienne and the consensus (which I share) is that it originally started life as a standard CQ case and has been remodelled (extremely well) to accommodate s strap. This could well have been a factory prototype judging by where it came from and the quality of the case work, I have quite a few factory oddity cases (including a few for cal 1510,1511,1516) which were of production quality but never made it to market.
Scary as it sounds back in the day Omega simply didn't give a monkey about old part or prototypes, staff walked out the factory with prototypes and no one batted an eye lid. Last time I was in Bienne I had a great conversation with a guy in a bar (who I might add runs a local watch should) who said that in the mid 80's Omega emptied the contents of the Megaquartz F2.4mhz spares in to a skip along with a bunch of other 'obsolete' spares and the locals went skip raiding.
A lot of my prototypes come from this region of the world, in fact my 1500 elephant (now on permanent display at STS) and my 1510 prototype round dial both came from John Othenin-Girard, the man who project lead the Marine Chronometer project.
I dread to think what is still lingering in locals sock drawers.