John, this type of design was used in Japanese military watches in the late 30's through to mid 40's.
Traditionally the military versions have a marked dial with an insignia indicating the branch, an anchor for Navy, a star for Army and a Sakura (cherry blossom) for the air forces of the Navy and Army, there was no separate Airforce. The exact group the Sakura models were targeted for has not been confirmed 100% so there is still some debate on this.
I have a Navy model that I acquired a few years ago.
The outer case separates and it contains a secondary case for the watch.
This inner case then separates to expose the dial or the movement.
As the original example does not have an insignia there is no way to tell for sure if it was or not a specific military watch or one purchased as a civilian. My guess for a date on this example would be 1943-44 due to the higher jewel count version of the movement but that is just a guess.
This double case design was originally patented by Francois Borgel. Throughout the later part of the teens and into the 20's they produced firstly "Hermetic" watches which had a screw down case back, and a screw down bezel but the crown was still external of the case.. later on produced double case with external crown like the above, and also "fully hermetic" where the entire watch case, with crown, was housed inside another case. To wind or set the watch you had to remove the bezel and the inside watch and case would hinge out.