Marketing. I guess not many knew that the Speedmaster came out before the popular 'moonwatch'.
The original was the CK2915-1 (cal 321) made in 1957 with the so called broad Arrow hands, borrowed from the Omega Seamaster.
The next model, CK2998-2 (cal 321) made in 1958/59 with dolphin hands and the MKII dial.
The next model, CK2998-6 (cal 321) made in 1958/59 with dolphin hands and black dial (moon watch dial) and luminous chrono seconds hand.
The next model, CK2998-4 (cal 321) made in 1958/59 with dolphin hands and black dial (moon watch dial) and non luminous chrono seconds hand.
The CK2915 and CK2998 are very rare and difficult to find.
The next model, ST105003 (cal 321) made in 1964. Looks like the moon watch we now know and love, with straight hands.
Personally I do like the Broad Arrow reference 3594.50, but I guess it is 'safer' to go with the 3570.