Is an R3/8" thread the same as BSTP? I have looked in many sources but the R threads are not positively identified as such. I have heard that this is a Whitworth thread but I haven't been able to verify that either.
I think the BSP thread is called "G" and is known as the parallel Whitworth. It is according to ISO 228, DIN 259 and BS 2779
I am not familiar with "R" but have seen "Rc", i.e. ISO 7/1 and tapered pressure tight connection known also as BS 21
R3/8 is ISO 7/1 series.. external tapered. See also BS 21, DIN 2999 and JIS B0203. It used to be "3/8 in. BSP.tr, EXT" It has the same thread form (55 deg angle and identical depth spec) as Whitworth.