I have several of those rifles here - had the same question - a 4BA cap screws turned in snugly, with fingers, all the way through - at least it did on the two I tried. Nominal dimensions of 4BA is .142-ish x 38 1/2-ish tpi, I think, so not so different that my fingers could tell. No clue what the +/- tolerances for screws and taps were. Dimensions shown come from Wikipedia - they use more decimal places... Without digging further, I do believe there was an "Enfield" series of screws - unique dimensions and thread counts, perhaps originally for or from Enfield's armoury, which, of course, is where the P13/P14 machinery came from, before being relocated to USAicon..