I don't understand why, other than to fit our theories, they would need to make castings of the T-Track. I would think there was an abundance of it -- there was enough for several Lightsabers in ESB --at least two of which still exist, three if we can assume the Barbican saber is part of the list. There was t-track available to the prop dept on RotJ as well for Boba Fett's blaster and, presumably, the shared Graflex Stunt (although I expect you could make the case that they just re-used the t-track that was already on the grip from before they converted it).