GINO raises a good point: So far as we know, a solid number of key ANH props did not have backups. There was only ONE Han Solo DL-44 hero used during filming, with the Greedo Killer quickly thrown together for post-production pickups. And apparently only one Vader helmet. One Leia Margolin-type blaster. Etc., etc. There appear to have been two Vader hero sabers (black and silver sidebars), but there also appears to have been only one Kenobi hero , since acquiring duplicates of all of those disparate parts to build more than one hero would have been a chore. It would have been much easier to machine a lookalike...which they did, with the two known stunts (V2 and V3).
And there was possibly only one Luke hero saber. We know that the production must have had access to at least five Graflexes, since various parts ended up on different props: Four clamps (Kenobi hero, V2 FX, V3, Luke FX), one Folmer thin-knurl red button (Luke FX), and at least one slightly modified full flashgun (Luke hero, likely a Folmer, but with full-knurl button replacing the stock Folmer thin-knurl). So, we can safely assume that the production may have acquired at least five flashguns, be they full or partial.
So far as we know, the only certain variation in the onscreen Luke hero saber is the placement of the upper half (which was apparently rotated for one shot, and also for the post-production photos). The prop was featured in one scene (when it’s presented to Luke in Ben’s cave), and then serves only as a belt-hanger, aside from being swapped out for the stunt FX in several shots during the remote-training sequence. It may well have been considered primarily as a costume prop (not unlike the droid callers and whatnot, which certainly didn’t have numerous backups), rather than a hand prop, since it only gets hands-on use in two scenes.
The Kenobi and Vader sabers surely got the most attention from the crew, with several FX stunt versions made to duplicate the heroes, since the sabers get a major showcase sequence during the duel.
Meanwhile, if there was really only one Luke hero saber, then it sure as heck wasn’t available for the POST-post-production Bob Seidemann publicity photos, nor the HOLIDAY SPECIAL.
The early Finse photos from ESB show a saber with seven ANH-type grips that apparently fell off from the intense cold. Was this the reworked ANH hero? Or an initial attempt to recreate the hero with a new flashgun (and some tweaks in design), which had its grips hastily replaced (and screwed on— or was it riveted?) on location?
Certainly, the production acquired a good number of flashguns for ESB, since Luke’s lightsaber is a key prop in the film that is featured in closeups and lots and lots of stuntwork, unlike ANH.