Looks like one of the early 1171's with that clasp. They started appearing from around 1970 but I believe were more commonly seen from 1971 onwards. You'd need to narrow down the production or delivery date of your Seamaster to see if it could have been original. I'd think it was mire likely to have a 1039 or 1506. Even if you go with the 1171 you'll need 20mm links such as the 633.