Cal 321 is in Speedmasters from 1957 until 1968/69 (all depends on the reference whether it could have a cal 321 movement or not for the watches made/sold in the later years)
+1 for Moonwatch
Those of us born after ‘69 (‘72 for me) are much luckier than you pre ‘70’ers - our birthyear watches are far less expensive.
But don’t sell the MKII, that’s a cool watch