I would hazard a guess that no matter what you get it will be a standard thread size, though it may be metric or imperial. Either way it shouldn't be too hard to find a nut to hold it in.
What exactly does the fitting go through? Shouldn't take too much to get it through sheet metal. [Note: just looked at my spares: They are going through a single ply of 18 or 20 gauge stainless. There is a thin jam-type nut in the inside. Didn't take it apart to measure it.]