I had/have a sunwayfoto clamp designed for the p0 which has served decently, but I find that it can come undone too easily, especially if (for example) I've got the setup strung over my shoulder. Don't need my tripod just falling off!
It seems the RRS clamp options are between B2-40-LR-M6 and B2-AS-II-M6. I've seen some people say the AS is too big for the head and can also interfere with the panning lock. What's the disadvantages of the B2-40? It seems slightly smaller, but the jaws don't seem much shorter?
My one worry about the B2-40 is that it appears to be an older design. Is the lever still good? Would it still be "flexible" accepting non-RRS plates? I read somewhere that their clamps at least used to not accept wide range of plates? I've got a Kirk L-bracket on my 5D Mark IV, so it's a newly-manufactured plate.