Re: "A friend of mine made a very good living doing M just after leaving University."
MUMPS was not Forth based, as it preceded Forth by about 5 years (1966 Vs 1971).
Although its terseness and design of breaking code into 2KB blocks is very Forth like.
OTOH variables <==> files <==> b-trees mean anything can be made persistent across all instances (IE a file) just by putting "^" in front of the name is not very Forth like.
And then there is the command abbreviation, combined with number of spaces between some of them being significant. That could make for a complete mindf**k when reading through old code, to the point of writing a tool to expand such abbreviations to make the whole thing more readable.