Looks like a reference 2640, produced from the late 40s to the ca. mid 50s. It was officially a Seamaster, although not all examples carried the model name on the dial. Have the caseback opened by a watchmaker, inside the case back it'll state the reference number, on the movement you'll find a serial number, indicating in which year it was produced, as well as the caliber number. My guess is cal. 283.
A nice find, and a beautiful heirloom. I'd recommend against using it before you had it serviced, but then this might be a lovely to wear, even on a daily basis.