So - I may have found something cool.
I dug through some american caplug sales things. (Some are used to plug test tubes, so I think these are meant to be used
upside down) The tapered shape seems legit to me, that doesn't look like a straight cylinder on the real prop.
Anyway - Kurtyboy had originally deduced just under 8mm for the diameter of the red button. 7.9 mm is 5/16 in the US, and I found one with a 5/16 top (or bottom, however you look at it.)
Originally I thought it was too short - however that is because my Larbel socket has no pins in it. I dropped it in there and it was barely over the rim.
The original Graflex pins are the key here - if you squish the conical plug
onto the pins it will sit rather tight and slightly at an angle just like the photos we've seen! You can't push a cylindrical button of that diameter around the pins, it would have to be really short. Also, this has a flange which helps keep it from falling out if the spacer is glued down.
Also, Kurtboy had measured 5-6 mm for the height of the mystery metal spacer. When this is stuck on the pins, it sits 5-6mm above the rim of the graflex! Mine just has "2 X" written on top....I bet I can sand it off, but I wonder if there are any without any writing on them.
here are pictures, including one with an
incorrect spacer - just a tad too short, it's 3-4 mm thick.