As previously mentioned, it is possible for a male BSPT (tapered) to thread into a female BSPP (parallel), so long as the female thread is fixed and not swivel (this is because the recessed cone seat could interfere with clearance). To identify a BSPP fitting complete the following steps...
parallel Metric threads is that an O.D measurement will reveal the actual thread size (i.e. 18.01mm measurement means the thread is likely M18). However, to do this accurately with Tapered Metric threads one must measure the O.D at the third row of threads from the end of the fitting....