I think this is new, or at least brings together comments made disjointedly.
Some months back Ruslan surprised us by finding a Mountevans champion, Colbeck I think, that pre-dated Joint Promotions. The ensuing investigations revealed that the first Mountevans had indeed been before 1952, the formation of Joint. I don't think we ever concluded just when, though we do know the rules were formed at the end of 1946.
Looking at an old topic on Red Kirkpatrick Main Mask posted an article on Kirkpatrick from January, 1951. I've only just noticed that below the article is a list of what appears to be the very first set of Mountevans champions, in 1950.
No logic Anglo, but it would be interesting to find when the Mounevans label was added to each of the weight champions.