50x was a somewhat standard number for double ended S-wrenches, it was likely part of a set of sizes. The 500 series was split when ISN's were adopted, probably because more than one type of wrench was using the 500 series numbers. (the 500 series ISN's became the standard numbers for Set screw and tool post wrenches)
I have a few Champion marked S-wrenches as well, I am not really sure who made them (Syracuse??), but lean toward them not being the same as the OK champion ones..