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For bellows use on the PB-4, the camera mounts to the bellows directly or with a lens mount converter, so camera mounting is not required. To mount the whole bellows to a tripod or other base, any short general purpose plate will work fine, as long as the thread size matches. I do not know if the PB-4 is only 1/4 inch threads, but most of the bellows from that era were. Really Right stuff makes short general purpose plates with either 1/4 inch or 3/8 inch screws, so just get what you need.
If you have any reason to move the whole assembly closer or further away from a subject, you may want to consider a longer plate on the bottom of the bellows. That would allow you to loosen a clamp and manually slide everything to help get the framing and focus you need. All the Arca Swiss manufacturers make longer general purpose plates with 1/4 inch screws, so Really Right Stuff, Wimberley, Kirk, and Henjar are all potential sources.