Follow up. Stand back -- I'm going to try Science!!
Hokay, first of all, I poked around until I found a very useful reference image. This is one of Gino's, taken at MoM, with laser calipers to nail down some size referents.
I cropped it and printed out the relevant portion, measured between the two closer dots, scaled it so they'd be exactly an inch apart, and this is the final version, as close to 1:1 scale as I can manage, really. There's some distortion due to the wrist end of the gauntlet being farther away from the camera -- that far dot is supposed to be six inches from the first one closest to the elbow end, but it's off by about a quarter of an inch. When I juxtapose it against Alan's pretty-darn-good template, you can see how the distortion gets worse toward the wrist:
Fortunately, we're looking at the other end.
Okay. The tubes inside and alongside. Alan was right in making them 1/4" in diameter:
Those hatch-marks along the edges of that ruler are 1/16" increments. 4/16" = 1/4". So that means the fittings I'd gotten that are optimal for a quarter-inch tube are literally the best fit, but how do they stack up against the fittings recommended in this thread?
1/8"-pipe female-to-female coupler (above) versus 1/4"-pipe version:
Here's how they look alongside, to compare relative lengths:
The hex knurl in the middle is a little wider than on the original pieces, but not egregiously so. The 1/4" coupling is a better match for overall length, but is off in the other measurements.
Here are the 1/8"-pipe and 1/4"-pipe close nipples (never get tired of that -- I am apparently twelve years old...), respectively:
Even the 1/8"-pipe one is too large in diameter. I may grind the threads down a bit to make it a better match -- leaving the threads that will actually be needed, of course.
Then the flare nuts. The 1/4" one is pretty darn good, compared to the 3/8" one:
Moreso when it's up on a corner, as the one in the photo is:
Even the 5/16" nut is significantly larger than the original:
I'll have to grind down both ends of the nut to make it a better match to the original part, but that's no biggie. A last view of the fittings I got mocked up in place and compared side-by-side with the photo of the original in the same scale:
After going through all that today, I think I'm going to stick with the ones I found. I'll keep y'all updated as I get things cut and ground down and actually assembled...
--Jonah