At 1/4" NPT and BSP threads are only 1 TPI (threads per inch) different so its very difficult to tell what you have unless you have an accurate thread gauge.
The angles of the threads are also different, one is 55 and the other 60 degrees
Also both NPT and BSP come in straight and tapered threads and the tapers are not the same.
With some force you can make a BSP male fit an NPT female, but how can you ensure it will be gas tight?
NPT is only really common in the US.
BSP is common in France and the UK...despite France being metric ... French plumbing is mysterious !
It is more than likely that the connectors on the boat are BSP.