This is exactly what you assumed:
A not too bad, but not too great amateur mod.
Epoxy instead of Pertinax circuit board material, and some metal film resitors used, point to much more contemporary than the original.
Some other parts look like old, but not from the C-28 era.
Tinning the board traces is unusual these days, and is obviously executed by hand.
Not too good an idea in the high impedance part (FET etc.) of the mic BTW.
Can make the mic humidity sensitive.
From the looks there’s no way to judge whether the BF245C FET was properly biased, or how the mic sounds.
The transformer at least is NOT suited for the purpose, it’s made for 300 to 3000 Hz band-limited 600:600 Ohm telephone lines, see the datasheet:
https://www.farnell.com/datasheets/2830074.pdf
