[Whaler]

Yes its a little different than what I expected. the nut and ferrule is separate from the 1/8 tube
You rely on the ferrule to bite the tube when the backing nut is screwed onto the gauge. Thus they are called compression fittings.
Yours sounds like a flat mating face with the backing nut used in the same way
Funny there's no bevel in the seal surface like in a female flare or compression fitting

I have Autometer "smiths replicas"

I guess you should just drag the gage with you into the speed shop

Sorry I couldn't help

Tim