A member on another board worked a Fox back then, and he said that it was a merchandising deal that Fox was trying to do that fell through.
The REMCO Okinawa Pistol also know as the Grenade Pistol, was used in two toy lines. In olive green and black in the "Monkey Division" line. The pilot rifle was the Okinawa rifle. The Grenade Pistol was finished in chrome plating and black for the "Hamilton's Invaders" line, this is the one I had. The H.I. version was used as the Tracker's LASER pistol in the movie "Cyborg 2087". In the late 60s, the Grenade Pistol was marketed as a "Star Trek" toy, I had that version too.
I think that there are so many variations because it was a toy, and the plastic was not up to the stress of prop work. So they made resin versions for different purposes, and only used the hero version when they needed to see it light up, or in closeups.
They had to change the design for the second season, because if they were going to market it, then they couldn't use REMCO's design as the base.
David.