Some more food for thought; When measuring from the end of the piston to the bottom of the bore of the pushrod cylinder it is only approximately 1/2" deep. That goes for both pieces that I took the pictures of. That isn't much room for a rod to fall out, or drop out and wedge up against the firewall. I see what Rick is talking about with the early power brake cylinders not having as deep a bore as the one's he sells, but I don't have a multitude of old master cylinders here to check. I still have my original one, before the upgrade, and it has the same bore dimensions (with the machined groove) as the one piece I got from autozone.
My thinking is that if the Chrysler Engineers put it there for a few decades, and on different models of Master Cylinders, than it stands to reason that the retainer, for non-power brake cars, needs to be there! I've read most all of the mopar mags, and have them dating back into the 80's when they first starting publishing them, as well as service manuals, guides, etc... Whatever mopar I read it, but I have never read anywhere that a fix for not having the retainer is to put RTV in it. I might have missed something somewhere along the line, and maybe this is an idea that this is a trick fix that Rick has become aware of, I just have never heard of it myself. As for myself, I have just spent years working to get my car back on the road, and I sure would hate to see all that time, money, and divorces wrapped around a tree because of a $1.00 piece of rubber.