The original RR Merlins were all Whitworth, or to be more precise BSF (British Standard Fine) for the most part, with a few BSW and of course BA threads chucked in as well!
As far as I can recall, the licence built Packard Merlins used exactly the same fasteners, in order to retain interchangeability in the field! The difference, of course, was that Packard, like Rolls, bought in many of the ancillary parts, thus a Rolls engine would have BS fasteners, as would it's SU carb. A Packard would have the same, but it's U.S. carb would have American National Fasteners, probably from the numbered series as well, and so on for all the other bits!
To be honest, this is the reason the British government suggested we all adopt the 'Unified' system, but it also explains why many British aero engineers, especially the old blokes, have a VAST collection of wrenches!
Incidentally, my Dad flew Merlin powered aircraft right up till the mid 60's! Although the engine is revered now, Dad reckoned it was fine for powering a bomber that would probably be lost after a couple of hundred hours, but no good for airline use! The main problem was the cooling system - quote "about fifty dodgy old rubber hoses, held together with a hundred jubilee clips, any one of which could fail and give you a shutdown"!