Rolex 300 is a manual winding form movement by Rolex.

Features 18,000 A/h, 15 jewels, small seconds, power reserve 50 hours.

Identical Gruen 877 (Alpina Gruen Guild).

Built by Aegler and used in the “Doctor's Watch”, Rolex used the Calibre 877 with slight modifications for the model Prince from 1932. The Prince was the first wristwatch produced in large numbers that received a chronometer certificate.