Nice pick up, T6.
Phil: Yep, Palmer Plastics.
The gun was available on the card with a dart, or bagged with a header card and dart. Every carded one I've seen has been black and white, with an almost airbrushed quality to the painted details. (Similar to what you'd see on something like the Tom Corbett rifle -- the masking wasn't very tight, I guess.) The cleaner paint job on yours, T6, suggests that it was done with a bit more care... Making me think it was probably a later, bagged examples.
As far as I know, only the carded version of the gun came with the exploding darts. The bagged version had regular suction cup darts. Palmer also sold the darts on their own in pairs on space-themed cards. (I've got a set -- I'll post a pic tonight.)
It's a simple gun, but I find it kind of charming.