Help identify fuel-cell thread.

[ErikLS2]

I'd love to help a fellow Dane out but it's hard to read the calipers. There isn't a common 19mm thread so if it's metric it's either 18 or 20mm but that 1.75 doesn't appear to fit in the threads like it should. Also, the fitting should have a shoulder on it for a sealing washer if it was that kind of thread. I'm guessing it's pipe thread, maybe it's even British standard pipe, depending on where it originated.