I have been using Mamiya RB Pro S and Graflex XL.
You can mount both Graflex knob and Singer lever holders onto Mamiya RB pro S. Both film advance and double exposure enable pins just clear the top of the housing and does not interfare with the camera operation.
Also you can mount Mamiya pro S holders onto Graflex and the camera works fine.
But there is a problem. The film registration is different. For Graflex, it is 0.190" and for Mamiya 0.173". This is the reason why Arca Swiss makes different adapters for those backs.
Nevertheless, I am quite happy using RB pro S backs on my Graflex XL. I put a shim made out of two layers of black construction paper (0.015") under the back. The Mamiya backs make quite a improvement on the film flatness over Graflex ones.
Unfortunately there is not a easy way to offset the difference on Mamiya with Graflex back. One may not notices the focus shift with a long lens specially close ups, but shure sees it with a wide angle lens.
The prices of Mamiya RB's came down recent years, and it is the much better idea to get the proper backs for the camera.
I hope this will answer your question.