Your Seamaster Calendar is an absolute cracker, the six o’clock date window gives it a lovely symmetry. What reference is it?
2757.
The other date-at-6 bumper is a 2627. You can tell the difference by the lugs - the 2627 had deeper cuts where the lugs meet the case. See the pics from @Tritium's and Norm's sale listings:
Either case could house the 353 or 355 bumper - both had the date at 6.
Also, you can tell from the trapezoid shaped date window that it's 1953 or later. The first 'generation' (for lack of a better word) had square windows from their introduction in 1952 until the switch to the 5 sided window sometime in 1953.
EDIT: forgot to mention the clover crowns were on the later generation, while the first generation had bowler crowns (but that's not carved in stone).