This is what it looks like the inside of a vintage Omega Electronic F 300 Hz wristwatch. Although this tuning fork movement was designated by Omega as a caliber 1260, the base movement is an ESA 9164 found in several other watches from Baume et Mercier, Eterna, Longines, Tissot, etc.
This particular watch was experiencing the dreaded spinning second hand issue when adjusting the time (you can watch a video of it below, and learn more about why that happens here). Enjoy the gut shot photo gallery!
The Accutron, an electromechanical watch developed by Max Hetzel and manufactured by Bulova beginning in 1960, used a 360 hertz steel tuning fork powered by a battery as its timekeeping element. The fork allowed it to achieve greater accuracy than conventional balance wheel watches. The humming sound of the tuning fork could be heard when the watch was held to the ear. –Wikipedia