..book (Marco Richon) gives the following information:
600 27.90SC PC AM 17P 1960 (manuel version of 550) Seamaster 409.000 pieces
601 27.90SC T1 PC AM 17P 1960 Seamaster -De Ville -Genève 831.000 pieces
602 27.90SC T1 PC AM bull 17P 1964 Constellation (rare)
This could mean that the 602 is essentially a 601 which underwent additional regulation at the factory in order to receive a bulletin.
Ben.