GTX said:I understand that Martin upgraded the aircraft to 2S standard in 1958-1961 so one would assume that window would be when they entered operational service.
Looking at some of the units that flew them we see:
VP-30 - aircraft entered service December 1962
VP-40 - aircraft entered service October 1960
VP-45 - aircraft entered service December 1962
VP-47 - aircraft entered service December 1962