Hello, everyone. I didn't receive any response to my recent subject post but have found the answer myself. I was only searching USPTO patent class 70-21 instead of including class 70-25. Under class 70-25, Master Lock patent #3563067 of 2/16/71 for an improved permutation padlock appears to describe the internals of the currently-marketed Master #1500/1525. I am assuming that Master began marketing the improved design in the early 70s.
The foregoing is provided for those like myself that are interested in lock company history and lock evolution (especially stuff still manufactured in the U.S. - yes, I remember Yale & Towne, Slaymaker, Corbin, National). Sincerely,