regarding the base case, movement, bracelet these 4 different B-42 watches (Diver, Cosmonauts, 2x Marinemaster) are same. For the newest version of the Diver even the bezel is same, the older divers did have a blue bezel. The white face Marinemaster stands out, because it has a different (silver colored) bezel.
The main differences are, as you noted, in the dial, day/date wheels and the hands.
The dial is black for the Cosmonauts and Marinemaster, with a white set of day/date wheels in the Marinemaster, black day/date wheels in the Cosmonauts. The hands on these 2 are identical. These 2 models with the respective ref. numbers 647.10.11 (Cosmonauts) and 647.10.41 (Marinemaster) even share the same case back number 647.10.158 because Fortis replaces the last 2 digits of the ref. number with a 3-digit movement code in the case back number. The Marinemaster has bolder luminous numerals/markers on the dial and a stronger blue lume.
The dial on the white face Marinemaster 647.11.42 is full luminous coated, with black numbers and hands, blue seconds hand, black day/date wheels. Silver bezel.
The Diver 648.10.12 has the most elaborate set of dial and hands, the dial base is a whitish opaline material, the numerals are applied blue steel numbers. These are true applied numerals, not just stamped out of the dial as is the case at some other brands. The hour and minute hands are semi-skeletonized, blued steel hands. This elaborate execution of the dial/hands goes at cost of the luminosity, only small luminous indices and luminous tips on the hands. White day/date wheels. Older versions of the diver did have a sweet metallic blue bezel, this has been canceled in mid-2007, newer watches do have the same black bezel as the Cosmonauts. Important : The B-42 Diver has a display back!
These 2 particular models (B-42 Marinemaster / B-42 Cosmonauts) are too similar. See the 2 pictures below:
If you want another day/date Fortis in the B-42 (42 mm) size, the white face Marinemaster could be a good idea:
Or the newer B-42 Flieger with black face, these do have the unique arrangement of the day and date windows - a direct comparison to the Marinemaster below:
Or with the addition of a GMT functionality, either the B-42 Cosmonauts GMT (left) or the B-42 Diver GMT (right) - but the GMT functionality adds a premium to the price…:
Or you leave the 42 mm size requirement and go for one of the older original “Official Cosmonauts” day/date in 38 mm size, these are: completely different in appearance, still being made, available new, and actually very nice little diver’s watches:
