I've always suspected that Packard used Ford's revised drawings rather than RR's although I've no grounds for it, it just seems logical.
I'm on the road at the moment so unable to give the exact difference between UK and US drawing standards, but the change was to bring the drawings into line with US engineering practice. Of course Packard also introduced their own mods, supercharger drive, mags and pressure carb that didn't cut out with -ve "g". One major issue facing Packard was having to manufacture dies, taps etc in order to keep the British threads on screws, nuts etc, which led to some delay in the program.