@Dave_Johnson I had these laying around, they are 4mm tube x 1/8" BSP thread I think.
But for using a nut on the back look for bulkhead fittings. There are a million brand names out there. But something like this:
@Eclsnowman BSP shudder
@Ryan_Carlyle You are telling me. Probably the reason I had them laying around. I likely pulled them out of something to switch over to something standard. I hate finding out I have something with BSP fittings. They get mixed into the fittings boxes and you only realize it once you have cross-threaded something trying to get them to work in an NPT port because of the slight change in TPI. But certain industries latched onto it for some reason. Just another pile of fitting I need to keep on hand 
@Eclsnowman The UK oil industry likes it, and we get some of our equipment built there, so it’s always a risk. I actively write fitting requirements into my specs now. At a former job I had a coworker put 1/4" NPT male fittings into a 3/8" BSPP female 10,000 psi hydraulic manifold once. (I think that’s what it was.) Held pressure with teflon tape, but it was surely a deathtrap. Didn’t find out for months. 18-tpi tapered threads in 19-tpi straight threads, iirc… the taper lets you get 2-3 threads of engagement without necessarily noticing the mismatch.
You guys don’t like sla?