Using the NKG part number guide available here,
NGK Spark Plugs USA
A BPM6A decodes as 14mm plug with projected tip and 3/8 long threads
The B6HS Is the same as the stock plug, 1/2" threads
BP6HS has the projected tip as well.
The shorty plug will leave exposed thread in the chamber, not good for many reasons. Especially when you try to install the correct 1/2" plug and the threads are full of carbon or burnt away.
The shorty plug may fit better without its washer or if the head is machined to accept 3/8" plugs. The quality of the alloy used here might not be good enough for only 3/8ths of thread though.
One of my engines blew out a shorty plug once.
A helicoil insert fixed the issue though.