I'll add my 2¢ to this.
The saber Luke wears on his belt, that features in ALL of the Elstree Studio shot principal photography is what has become known in Fandom as the V.2. It was a left over Obi-Wan Kenobi stunt from ANH, which had the blade cut off resulting in the ROTJ saber "nipple". This saber was cast aluminum and is irregular in the extreme, not only in its ring positions and angles but in its overall cross section which is less than round in many places.
This prop features in ALL of the studio shot scenes. It is the saber handed over to Vader on the landing platform. It is the saber in the Emperor's hands AND on his chair in the throne room scenes. It is the saber Luke throws away when tempted to turn to the dark side. It is NOT the close-up saber from the "I see you have constructed a new lightsaber" scene.
That insert, shot after principal photography had wrapped, was filmed in the States. From what MR were able to ascertain a cleaner, prettier machined saber was used in the hand roll-over scene. If you freeze the frame you will see that its control box doesn't resemble what we have all called the "HERO" version, as it has simple copper/brass angle on the top face of the box and not the more elaborate step which seems to feature on some of the "HERO" sabers.
What we all known as the ROTJ Hero saber, was in fact the machined stunt saber, used for all of the dueling/fighting scenes shot in the US. It is still unclear if this saber ever saw use in the UK for the final duel btw Luke and Vader as in that scene it looks like they utilized a second, bladed Obi-Wan ANH stunt.
I have pics that some folks here also have that show Mark resting his chin on what is clearly the "HERO" but with a missing control box top and a short blade attached to it. It makes sense that the fighting props should have been machined for the exact purpose of fighting.
In Yuma and in Crescent City, Luke continues to wear the V.2 on his costume and it is this saber he catches on board the Sand Skiff before he ignites it. After he does so it becomes the machined stunt saber complete with short blade.
Luke wears a different version of the V.2 during his climb up the side of Jabba's Barge. This V.2 features a wholly different paint job that is more consistent with it's look in ANH, AND has a standard Graflex clamp and lever but with the holes drilled into it. in the same positions as what we now come to think of as the "shared stunt" saber. I have a very clear picture of that.
The "I see you have constructed a new lightsaber" close-up saber seems to be a machined stunt, but with a different control box. The box lacks the inward angle at its base and the previously mentioned copper/brass edging lacks a step.
As micdavies pointed out in his first hand account, there were MANY copies of the saber for the firing from R2's head sequence. You never hold up production looking for one saber to reload when you can cast multiples.
The saber scene when R2's dome hatch opens, lacks the ROTJ "nipple". Was this cast from a completely different saber? Or was the nipple simply ground off the allow the saber to better fit beneath the hatch? I can't answer that one. My investigations never got that far.
My beliefs are based on what we could research from the archives.
The Endor scenes were shot early in production of ROTJ. The saber Luke wears on his costume is the V.2 complete with the ESB connector board edge strip in the top of the control panel. Lonepigeon turned up a great pic of Logray holding it that clearly shows it there. It may also have had TWO side bars still intact at that time, however the lever in the Graflex clamp was NOT the standard one. This had been replaced with a longer lever that has a more rounded profile (the confusing bent nail theory).
At some point during production the connector board edge strip fell out and the saber lost a side bar.
I personally theorize that the V.2 (or one of them, at least) was altered post production to look more like what is now held to be the "HERO" version. A detailed examination of this prop reveals that it was once painted in the Obi-Wan ANH colors. They're evident all over the prop plus the surface of the emitter has been ground down at an odd angle, to remove the nipple perhaps? But you can clearly see the black paint pitted into the surface of the metal that used to represent the circle of holes on the ANH stunt.
As for what Elstree Props have that are bona fidé? I have no clue. They had a picture on their site looked like the V.2 so I called them to enlist their help in my initial research. They were less than helpful and their recollections seemed inconsistent. When they misrepresented my conversation with them to a third party who is a member of the board here I decided to not pursue their "assistance". I subsequently learned that the "V.2" on their site had been made by a board member here. NOT that they ever misrepresented it as the genuine article, nor did they volunteer that it fan made.
How many sabers were used during the production of ROTJ? I have no honest idea. I arrived at my research for the MR V.2 by combining through archival photographs, examining the remaining props in the Archives and with some eagle-eyed assistance from Lonepigeon.
Barry.