I just wanted to add a little to the otherwise excellant guide by George Arthur Lareau that wrote on the used Mamiyas.  He said that the RB67's were all pretty much the same.  As for the camera that is true but there are some minor differences.  The Pro-S model has some interlocks that the original RB67 does not.  These prevent accidental exposures.  With the RB67 you wind the film and then with another lever you cock the shutter.  So in the Pro-S you are supposed to only be able to take one exposure per cocking of the shutter.  You can take a double exposure by turning a little lever on the film back but that is done on purpose.  The RB67SD takes two other lenses than the others and I think that you need an adapter to use the old lenses on this camera.  I'm not sure of that so check that out.  Only the Pro-SD model is made now.  Otherwise it is a great camera and you probably won't be sorry you bought one if it is in good shape.  Expect to either have the light seals replaced, or you can do this yourself.  There is info on the net for doing this.  I bought some felt from Wal-Mart and did it myself.  No big deal.