Nice project!
Don't lose those chromed-screw-with-plastic-half-moon Huret rear dropout adjusters! If your '73 is like my recently-pressed-into-service '72, then the holes in the dropouts are unthreaded (and larger than will allow tapping for a typical Campagnolo-style adjuster). The thread on the screws is non-standard too: M3 x 0.6 rather than M3 x 0.5. The plastic half moon is the "nut" as you have probably found out. While it's possible to put together something that works with a nut on the interior side of the dropout (using a typical screw), I don't think you will find replacements easily for the originals, unless you buy another frame with such Huret dropouts. What does work (if the screws are bent or break) is that the half moon will accept a M3 x 0.5 screw, or so it seems to me -- I mocked it up (just wanting shorter screws) and they seemed solid enough that way, but because my frame has gotten a Sturmey-Archer AW drivetrain, decided the adjusters were more trouble than they're worth (nutted installation, need to adjust the chain tension like any derailleur-less bike).
I know -- TMI.
Oh, and if your Jubilée RD ever dies, then you might be interested to know that the RD on my Competition had already done so (or been sold), but it came with a Shimano 600 "Arabesque" derailleur that had some sort of (looked like factory equipment) washer thing that made it work with the Huret "5 o'-clock" stop. I know there are other kluges out there for this predicament, too.
https://www.bikeforums.net/classic-v...r-options.html