The Hamilton caliber H-12 is a regulator style automatic movement with 25 jewels and a power reserve of about 38 hours. The H12 is a 3-hander no-date, made in Switzerland. Although the rotor is signed Hamilton, the ETA caliber number can be found under the balance wheel.
In Hamilton’s own words:
“The H-12 is a regulator movement that allows the minutes and hours to be separated from one another on the dial for maximum practical impact. Favored for watches used in situations where minutes are all-important.” –source
Hamilton H12 VS ETA 2825-2:
The H12 is a Hamilton branded version of the ETA 2825-2. Hamilton and ETA are both owned by the Swatch Group conglomerate of watch brands. Currently, ETA movements are only sold to brands within the Swatch Group. The 2825-2 that this movement is based on can also be found in other Swatch Group watches such as Tissot, with different variations such as a date and different hand layout. The 2825-2 is based on the 2824-2 framework.