After consulting with some very old repair people in the industry I have managed this: There is a mixer on board and an "FM" switch. But apparently there was an auxiliary unit that plugs into the mixer and the "FM" is to activate that unit. That was not at all obvious. I have no idea what this unit looks like or what it's part number is but I am pretty sure I am better off cooking one up on my own along the lines of what you have detailed.

I am giving you the thumbs up. Based on what I discovered which I will detail

@ChrisStratton I used the schematics and the part identifiers on the actual parts themselves to identify what was expected. In the case of the tubes finding compatible parts was tricky but I managed to find modern equivalents that exceeded spec. Some I had to order from former eastern block countries

@RussellBorogove - it is model year 1947 which is 2 years after the cut-over to modern(ish) FM range.

@chu update the description with the model