I have a couple of each and know they all fit the 500 series. Don't know about the flex but would thing it should.
One thing to watch for in the oldest quick releases (square type) is that the threads are sometimes messed up. On one of mine the threads were worn and I completely messed up by tightening too much. The material seems like a softer aluminum than used for the taped releases.
I find the long one a tad easier to mount the camera, but the short ones to be easier to remove the camera. In the end, though, it doesn't matter that much.
Good morning, Brian;
See if a "HeliCoil" can be fitted into the stripped threaded hole you have on your plate. This is a diamond shaped stainless steel wire that is wound into a spiral and when screwed into a special threaded oversize hole made with a special tap for this purpose, it restores the threads to "better than new" original condition. I say that it is better than new, because the likelihood of it ever stripping again is greatly reduced, and the fastener threads in and out so easily. Some engine manufacturers, such as BMW, use these in their aluminum engine cases from the factory to ensure longevity.
If the standard length HeliCoils in your size are too long, I have successfully used a small cutoff wheel on a Dremel tool to shorten the HeliCoil to the length needed.
And, yes, these actually are UNC thread specification devices, and, if we look critically at the specification, no, it is not the original 1/4-20 or 3/8-16 Whitworth Thread Specification, but fortunately it is very close, and should not pose any problem with our tripod screws. What is a silly 5 degrees in thread form angle between friends, anyway? Sorry, but I have not yet found listed in any of the catalogs HeliCoils for Whitworth threads. UNC, UNF or SAE, and Metric, but not Whitworth.