Took a punt with this 'Spares or Repair' RB67 Mamiya 120 Roll Film Back for £25 delivered, it obviously needs new seals and apparently 'the double exposure interlock plate (the bit that tiny pin hits) seems jammed in and some minor corrosion also the catch that holds back together is stiff.'....And indeed it did arrive in a mint original box in a grubby mess.
2 hours later the old seals are gone, the pin I think he was talking about was cleaned and pushed in and out a few times, the door catch was loosened off by using it a few times, there was an interesting screw that holds the top on which I luckily got out without too much stress and already ordered replacements as all will be replaced because all 4 are corroded!
After a good clean there's no corrosion on the body or on the film holder, there is some it seems inside the top cover but when I take the top off next week I'll clean and lubricate as needed but until then....Put it on the body and the interlocks work, the dark slide pin is a little stiff but looks an easy fix but it does indeed work!