In a nut shell, they are almost the same. A BSP will screw into an NPT hole and an NPT will screw into a BSP thred although there will only be a couple of threads with full engagement in both instances (i.e thread will not be of full strength).
The only difference is the flank angle on the thread form. If my memory serves me correctly (poof, blow away cobwebs from my hydraulic days) the flank angle on a BSP is 60 deg and NPT is 55 deg.
If it's yank then it'll be NPT although BSP fittings are easier to obtain over here. Good hydraulics shop will have both.
Are the fittings under pressure (I mean anything above about 100 psi?) as gas is far more searching than a fluid so will need a getter fit.
Hope this helps, sorry to waffle. If you want me to check flank angles for you let me know and I'll dig out mt thread charts.
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