They can vary and the position of each size can vary; the problem is getting a plate that fits the base dimensionally with the right size screw in the right place. I have 3 different plates.
Thanks, guys. My camera only has a single tripod hole. I verified it with the 1/4"-20 from one of my other plates and, although it threads, there definitely is a bit of slop. 3/8" it is.
I called Really Right Stuff (really great folks) and Emily helped me out, modifying my order.
The threads on my 1/4 x 20 are worn out so had to put a 1/4x20 threaded insert in the 3/8" hole. Did not know they had many styles of plates for the 500 CM.