My RWD experience does not cover anything earlier than a 1922 5HP model but, based on what I have seen, early Citroën threads appear to have always been “standard” in that modern ISO fixings are compatible.
Some head sizes can vary over the years but are always metric – for example I have come across 7mm screws and bolts with heads of 11, 12 and 13mm across flats, very often forged with chevrons on the top, so clearly original.
B……
Bernie