Well, it took a bit longer than I intended but the results are in.
To summarise;
- US Stanleys used 60 degree threadforms (UN or UN)
- UK and Aus Stanleys used 55 degree threadforms (Whit)
- Records mostly used Whit; BUT there appears to be a short period where UN threads were in vogue....
In the end I only bothered with checking the frog securing screws, the handle and knob studs and the lever cap screws as these were easy to get to and crucially two of them are the odd 12-20 size that sparked off this and several other threads.
US Stanleys:
- Late 19th/early 20th century 66 beading tool
- Type 11 number 5
- Sweetheart era 3,
- Sweetheart era 10-1/2
- Post WW2 51
UK/Aus Stanleys:
- 1950's 4-1/2 (UK)
- 1970's 4C (Aus)
Records:
- Pre-WW2 05-1/2
- Pre-WW2 04
- 1952-1955 04
- 1952-1955 06
- 1952-1955 04-1/2SS
- 1957-1967 010-1/2
- 1979 04 (ex-MOD and date stamped)
The Records were the strangest because the 1952-1955 04 and the 010-1/2 appear to use American threads. The round-topped frog ceased production in '57 and the ogee-frogged 010-1/2 ceased production in 1967 so there may have been a short period in the 50's where American threads were in use. If so then it couldn't have lasted long.
Despite using my 6X head magnifier, strong light and quality gauges I cannot be 100% certain that my findings above are correct; they were originally manufactured to somewhat generous tolerances and some of the fasteners I looked at are over 100 years old with all of them showing evidence of age and sometimes wear. The studs holding the handles were the worst to inspect as they had the poorest quality of manufacture and were also subject to the most damage in use; every rear handle stud had the end threads slightly stretched where they had been fitted into the base castings. I'll settle for 99% though.
There are those who say that 1/4" and smaller threads are pretty much interchangeable; despite this being a nails-down-blackboard statement if your tolerances are questionable enough then for light applications it can be tolerable. I can confirm that experimentally swapping fasteners between planes as part of this research didn't cause me too many cringe moments; the handle studs are so sloppy neither threadform made any difference!
So in conclusion I would say that for plane restoration purposes it is unlikely that mixing threadforms will cause issues. I would attempt to buy the correct threading tools for the expected threadform but either will work to get the tools back into working condition.