Mamiya has released three versions of the RB67. The basic formula is unchanged, only minor things changed in each revision.

RB 67 Pro (Original) 1970

• 3lb 8 oz without a lens
• X flash sync up to 1/400 sec because the camera uses a leaf-shutter–an iris shutter in the lens rather than in the body. However, 1/400 sec is not just your max sync speed, it is also your max speed, period.
• Removable backs for 120 film and 220 film that can be changed mid-roll
• Revolving back, instead of rotating the whole camera to get a horizontal composition you just rotate the back.
• Macro is feasible with most lens due to the bellows focusing, 1:1 possible for wide angle lens.
• Mirror lock up mode–you’ll probably need it because the mirror is huge

RB 67 Pro-S 1974

Same as Pro, plus

• double-exposure interlock
• viewfinder landscape/portrait orientation indicator
• self-erecting focusing hood
• Focus lock
• some later models have larger light baffles to allow for 6×8 backs.

 RB67 Pro-SD User Guide  from mamiya.com

Mamiya RB67 Pro-S Repair Manual

Mamiya RB67 Pro-S Parts Catalog

RB 67 Pro-SD 1990

same as Pro-S, plus

• Larger throat needed for two KL lenses 75mm shift and 50mm
• 6×8 backs
• Has larger light baffle to allow for 6×8 backs.
• Due to the larger throat, it is recommended to use a special spacer adapter to firmly mount Non-C and C lenses.  However, Mamiya indicates that these lenses can be mounted without the ring adapter.

Manual from mamiya.com

Suggestion: Buy a Pro-SD unless you find a particularly great deal. The price difference does not seem to be that great right now.