@B.Wilson roots of ⎕fmt seem to potentially head back to an APL/360 library function - I helped answer question a little while ago (stackoverflow.com/questions/78168626/…) which referenced DFT of which ⎕FMT seemed to be a variant
On a more trivia note, ⎕FMT was first main block of interpreter code Geoff wrote, believe even before the scanner, whilst Scholes was writing the parser
