[Lynn Eades]

Stew, there is more to this, and it is the evolutionary aspect
Vickers Carden Loyd built the predecessors to the carriers we know. VCL was a British company, who would naturally use British engineering standards. Pre war they were working with the French, in development work. I cannot recall for sure and could well be corrected here, but I think that is where the metric sized plate came from. If not, It is where VCL sourced their plate from.
Many things have rolled on through. As an example, the petrol cap on the Carrier fuel tanks was the earlier engine oil filler cap in the Vickers tanks. The Vickers tank track Became the standard from which Australian carrier track came, which later became the basis for Windsor track. I would suggest the Americans had a close look at the construction of the Australian LP1, and transferred features to the T16.
The metric steel plate went right through the carrier family (as far as I know)
If you want to check into the threads further, from memory the British thread will have a 55 degree included angle with a flat root, and radiused crest, while the American threads will typically be 60 degree included angle with a flat root and crest.
As Paul has eluded to, BSP ( particularly tapered) is a a very common thread form used all around the planet, more common that NPT.
Michael R has cleared this up for me, as I had previously disregarded BSP as a contender for the axle thread.
This little line is great:
The greatest cause of non observation, is preconceived opinion

Here is another thing to think about. The British inch, and the American inch were different lengths until about 1941, because they each arrived there by different methods. (If your machinists book has a green cover, Stew, it might be in there. mine is about 5 or 6 thousand kms away)