Just throwing this out there as a theory, but consider the Han ANH blaster that was recently passed off as original on Pawn Stars. Anyone can see that it has serious inaccuracies and some obvious replica parts, though it's possible that the scope is authentic.
Is it possible that Kurtz had an original ROTJ MGC Sterling that had been stripped after production, then got a hold of a Marco replica, took the pieces off of it and mounted them (incorrectly) to his base Sterling in order to make it look more like an E-11 for the sale? Still not completely honest, but it could explain why the scope is on backwards. Marco certainly wouldn't have made that mistake and I wouldn't put it past old Kurtzy after what he's pulled.