2. I would expect an old gauge to have pipe threads.
I agree. That gauge sure looks like a 1/8 NPT thread to me, but hard to tell for sure from the photo. Have you checked with a caliper to confirm it's straight? That would be unusual.
The taper is pretty slight, only 1 in 16 on the diameter.
NPT thread dimensions[on edit]On further study of the picture, it may be straight threads and the item on the left is the mating fitting for the dump valve for a compression test gauge. What are the threads on the other end of that fitting? Probably more standard than the gauge. If you can adapt to the opposite end of the fitting, You could drill the check valve out of it and plug the dump valve. Then it'd just be a dumb coupling/adapter.
[on edit again]If the gauge thread is indeed 1/8-27 straight pipe thread, then
here's an item that would adapt from 1/8 NPSM to 1/8 NPT. It's a swivel adapter, but would work. 1/8 NPSM (National Pipe Straight Mechanical) and 1/8 NPTF National Pipe Taper Fuel) are the same OD and pitch, except for the taper.
