BSPP vs BSPT vs R and Rc thread is different applications of adapters. BSPP is parallel thread/whitworth, BSPT, R and RC is taper thread, but RC is female BSPT thread. BSPP and BSPT (in most cases) have different threads per inch (TPI) (and consequently pitch).
The male BSPT taper thread will mate with a female BSPT or BSPP thread, so this situation BSPT can fit BSPP.
BSPT (British Standard Pipe Thread) is similar to NPT except for one important difference. The angle across the flanks of threads (if you sliced the fitting in half long-ways and measured the angle from root to crest to root) is 55 degrees instead of 60 degrees as it is for NPT. Thus an NPT male will fit into a BSPT fitting or vice versa but they will not seal. This is a popular fitting in China and Japan but is very rarely used in North America unless the equipment to which it is attached was imported. Thread sealant is needed to seal the male and female fitting together. Ralston Instruments has adapters to both male BSPT adapters and female BSPT adapters.
BSPP (British Standard Parallel Pipe) is most popular in the UK, Europe, Asia, Australia, New Zealand and South Africa. It is a parallel thread fitting that uses a bonded seal ring to do the sealing. This bonded ring seal is sandwiched in-between a shoulder on the male fitting and the face of the female fitting and is squeezed in place. BSPP pressure gauges have a longer male thread and use a copper crush washer that is squeezed in between the bottom of the male fitting and the bottom of the female BSPP hole forming a pressure tight seal. No thread sealant is needed to form a seal.
BSPT is British standard tapered pipe for pipes and tapered thread, also called R, R = BSPT = PT.
Thread codes and standards relating to BSP threads:
G - inch pipe thread (standard: BS 2779, BS EN ISO 228-1, ISO 7-1)
BSPP - inch pipe thread (standard: BS 2779, BS EN ISO 228-1, ISO 7-1)
BSPT - Inch taper pipe threads (standard: BS 21, BS EN 10226-1, ISO 7-1)
R (Rp, Rs, Rc, R1, R2) - Inch taper pipe threads (standard: BS 21, BS EN 10226-1, ISO 7-1, DIN 2999)
R thread Specifically as follows:
Rp - Inch sealed cylindrical internal thread
Rs - Inch sealed cylindrical external thread
Rc - Inch sealed taper thread
R1 - Imperial seal cone external thread, and Rp with the use of, that is, Rp/R1 "column / cone" fit
R2 - Imperial seal cone external thread, and Rc with the use of, that is, Rc/R2 cone / cone with