From what I've been able to glean, there are two movements for 1950s Date@6 Seamasters: 353 and 355. Is that correct? Are there more than those two? What is the difference between the two? Thanks all!
Yes - the early models from either caliber had square date windows while the post 1953 models had trapezoid-ish shaped windows.
you can see lots of different date at 6 watches here http://omegaforums.net/threads/got-a-date-at-6-seamaster.4378/page-3#post-50879