Allright - mixed opinions here.
The reason to why I was asking is simply because I wanted to know if I had the opportunity to do 6x6 without cropping etc. I do have other 6x6 cameras - folders and TLR but I really love the work I am able to produce with the RB - and the project I am doing at jeg moment involves both 6x6 and 6x7 and given the fact that the RB67 has removal backs it would be really awesome to shoot the series on the same camera.
About vignettes.. Improper lens hood could be an interesting thing to do - ill look into that I think.
I have held both RB67 and Graflex 2x3 Speed Graphics graflock roll film backs in my hands. But never both together.
I haven't tried this, so I may be completely wrong. I suspect that the later model Singer RH12 lever wind backs do not fit on the RB67, probably causing trouble with the RB67's interlock mechanisms. (I'm fairly certain the earlier Graflex knob wind ones do not fit.) But I also suspect that the Singer lever wind RH12 insert will fit properly into the body of the RB67's film back. I know that the guts of the Singer 6x6, 6x7, and 6x9 Graflock backs are interchangeable and you can get them mixed up. So, if it does fit, then you would get 12 exposures on the roll with the Singer guts inserted into the RB67 film back.
Without it in front of me I cannot remember where the image masking gate is located (on the back's body or on the insert), so I don't know if it would overlap the image or not. If the mask is in the wrong place (and I think it is in the wrong place) then a bit of judiciously applied Gaffers tape will probably work just fine to get a 6x6 image gate. You could make some slight marks on the RB67's back to guide you in placing the Gaffer's tape if you have to remove and reapply it regularly in the context of the project. And you really ought to have a roll of Gaffer's tape in your kit anyway. It's wonderful stuff.
Proceed on this at your own risk. I have never tried this myself. But do let us know if you try and it works or doesn't work. More knowledge is better, even if it is the knowledge that it doesn't work and the next poor soul shouldn't waste the time!
It's probably cheaper than trying to find one of the rare albino RB67 6x6 film backs. And if it works it would let you use the same camera for both formats for the project.
MB