Heck, even earlier than that. The phaser rifle was built in '65, and there's no telling how old any "found parts" might have been.
Does anyone know for sure WHO made the phaser rifle? I don't recall ever seeing any information from the usual sources about that particular piece. Laser pistols, communicators, and tricorders were all made by Wah, and the studio prop guys made the series phaser pistols, but what about the rifle?
Could it have been a rental from somewhere else? That might explain why it doesn't seem to fit, style-wise, with anything else Wah Chang or Matt Jeffries designed and why it only showed up in one episode. It's always looked a lot more Lost In Space than Star Trek to me...