6084--784,k SN, ca. early 1950s, "Semi Bubbleback" Chronometer--posted by tomvox1

Note: "The earlier BB has a fully sealed movement, meaning you will need to remove the top rotor set plate in order to work on say adjusting the +/-: calibre 6xx.

For the later movement, there is a small part of gap allow for that adjusting, an improved version, calibre 7xx. and because this half-sealed movement, many called it Semi BB." - Chin