What an answer! And from a man who actually worked for marlin!
I have actually seen a 336 with the "partial oval" you described. It almost looked like a big letter "C" encompassing half of the "JM" mark.
I really appreciate the effort Sir, truly I do.
Well, it was really no effort on my part. I do like to find answers to questions. Tom did say, in a later PM that Marlin did change to a different vendor around the time the cross bolt safety came out, but when exactly he could not remember. Besides, what difference does it make, we have the answer.