From my understanding, all the rotating back adapters for ProS and ProSD models supported the 6x8 frame size. You must, of course, also have a ProS or ProSD body to have the 8x8 opening in the body itself...
Oh, just realized, you're probably looking for the roll film magazine, not the rotating adapter.
As I understand it, the RB67 Pro SD came with a rotating adapter that was already compatible with the 6x8 back.
The RB67 Pro S may or may not have been sold with the necessary rotating adapter, but in their day a new rotating adapter could have been purchased and substituted.
The problem is that the rotating adapters can be moved from camera to camera, so the OP may have an RB67 Pro SD where the rotating adapter has been switched with an older, non 6x8 version, or may have a broken 6x8 rotating adapter on their camera or may have a camera with no rotating adapter at all.
From my understanding, all the rotating back adapters for ProS and ProSD models supported the 6x8 frame size. You must, of course, also have a ProS or ProSD body to have the 8x8 opening in the body itself...
Oh, just realized, you're probably looking for the roll film magazine, not the rotating adapter.
No. I'm looking for the 6x8 rotating adapter, which is not the same as the 6x7 & 645 adapter. It's a unique part. And yes, to fit my recently purchased SD.
On further investigation things become even more complicated. According to instructions, I see that the light baffle between the camera body and the adapter must be changed to one that is marked "8" or "6 x8". The instructions read, "Mamiya Service Center will do this improvement for you." Now I'm not sure if I need a different adapter as well.
Did your ProSD come without the rotating adapter? Or do you have reason to believe it isn't right? The original parts on the ProS and ProSD all supported 6x8; those required changes are only applicable to the original Pro model, as far as I'm aware. And those light baffles are far less likely to have been swapped than the rotating adapter, so the only one that has much likelihood of being too small on a ProSD is the rotating adapter -- and if you post some detailed photos, I can compare them to my ProS parts to be sure you have at least that new a version.
On further investigation things become even more complicated. According to instructions, I see that the light baffle between the camera body and the adapter must be changed to one that is marked "8" or "6 x8". The instructions read, "Mamiya Service Center will do this improvement for you." Now I'm not sure if I need a different adapter as well.
Wow! With your help I think I've got it all working. MK, you are right that those instructions were for the PrpS and not the SD. I won't need another adapter after all. Indeed, there might not even be one. Thanks!