The 'MoM Hero' and the 'Real Hero/Vader handled saber' (we really need a name for this one) do seem to match in overall dimensions including the uneven grip spacing. Either they were both made at the same time and detailed differently or the control box was changed. I'm guessing they are two separate sabers because as you said the R2 sabers have the detailed control box like the 'MoM Hero' and they were used at the same time as the 'Real Hero' with the very solid, simpler control box.
It wouldn't make sense if they cast it with the detailed box, changed the box to the simpler one for filming, and then changed it back - not impossible, just seems unlikely.
So I believe that the "I see you have constructed a new lightsaber" prop and the MOM metal saber are the same prop, but that that control box was modified at some point in post to have additional details on the inside (and that this is when the grey rails were added) so that they might be able to shoot the Luke constructing his lightsaber scene (which was never actually shot).
Now the only problem with this idea is that the resin sabers we've seen at MOM (which we assume are from the same batch as the resin sabers that were shot out of R2) have the same control box as the MOM metal saber, which would mean that the MOM metal saber's control box would have had to be on the saber early in production.
However, what if the resin saber shown at MOM is not one of the kind shot out of R2, and what if that resin saber is actually just something made later or even in post production.
What if the resin sabers shot out of R2, were actually castings of the Shared Stunt saber (the type that is on display in the entrance to the Lucasfilm Ranch).
Now some people will say, "but we've seen the resin Luke saber that was shot out of R2", and I would say, "have we"? Elstree sold a resin Luke ROTJ saber last year that they claimed was shot out of R2, but I think they are full of $hit. I think that elstree bought a resin casting of the Luke ROTJ saber, the same kind that were sold the the public in the early 80s (and which a number of board members own) painted it up, cut the tip off of it, and then passed it off at auction as the actual prop that was shot out of R2. But I think that we actually have no idea what the resin saber shot out of R2 in the Yuma shoot looks like, and that this saber could very well be a resin casting off of the Shared/Stunt saber, and that it could actaully be the kind that is sitting on the wall of the loby of the Lucasfilm ranch.
So this leads me to construct a new timeline for the metal "hero" saber in which:
A couple of Metal "hero style" stunt sabers were used at Yuma during the sandskiff shoot, one of these stunts (not the one shown resting under Mark's chin) was later cleaned up and used in the "I see you have constructed a new lightsaber scene", then this same exact prop was modified to have a more complicated and hollow control box , and this is the prop seen at the MOM exhibit.
I think that the resin saber at the MOM exhibit might actually have been cast very late or post production by ILM staff, after all of the modifications to the control box had been made. And I think that everything Elstree says is a big pile of crap.
Dan Stokes
DDStokes@aol.com