But close enough that I hope the folks here will excuse my posting it.
This is a WWII-era 35-size Hamilton Model 22 deck watch. They often turn up in gimballed chronometer-like boxes, but also show up often enough in a standard watch style case, too.
These were used(I'm told) as a deck reference for the time set on the main chronometer(usually a Hamilton model 21) kept below deck. After the war, they were cataloged and advertised as a master reference for jewelery stores and the like.
Here's the back, showing the ordnance marking
And, unscrewing the back reveals...another back
The second back pops off to reveal the movement
The movement is a standard 21j lever, not unlike its smaller cousin the 992B(the Hamilton 21 is a more conventional chronometer, with a detent escapement and fusee).
This really is a massive piece-something you don't appreciate until you have one in-hand. Here it is next to an 18 size pocket watch, to give some idea of the size.