Commonly known as the "天測時計(Tenshoku Clock)" issued to Zero fighter pilots of the Imperial Japanese Navy during the Pacific War (World War II).
The engraving on the back cover is "空兵 第八二五三號". I think it means "Airman No. 8253.
On the back of the back cover, you can also see an anchor mark indicating the Navy.
Here is a link to a video about this watch in Japanese, and a link to this watch sold in the past.
https://curious-curio.jp/watch/11834Click to expand...