Here's the history of the SWC model, data from:
1954-1958 SWA & SW
"Superwide Angle": fixed Carl Zeiss Biogon f/4.5 38 mm wide angle lens.
SW: 1956 slightly changed lens barrel construction.
With silver C lens.
From 1969 with black C lens, from 1973 with T*
Successor to the SWC. Ability to attach a polaroid back.
From 1982 with CF lens.
From 1985 onwards: spirit level in new style finder, removed from body.
1988-2001 903 SWC
Successor to the SWC/M. New Palpas coating in the body to reduce flare.
2001-2006 905 SWC
Successor to the 903 SWC. The Biogon had to be recalculated for new environmentally friendly lens materials. Now comes as a CFi lens.
My first Hasselblad Super Wide was the 903SWC. Sold it in 2004, missed it over the next eight years, so I acquired another. The second is a 1978 SWC (black lens, T* coating). The performance from them is so very close to identical... I prefer the styling of the earlier model and the way its DoF indication works.