I have two stories of its origin,
One) It was designed by Canadian designer Sid Bersudsky, and manufactured by Peter Austin Mfg. of Toronto. It was also licensed to Rainbow Crafts Inc. of Ohio.
Or, Two) It was designed and originally sold by Rainbow Crafts and licensed to Peter Austin Mfg.
There isn't much info on the Peter Austin Mfg. company except that it was a division of the Kelton Corporation of which there is also little info available, though I found the company registered the Magnajector trademark in the US in 1954, giving some evidence for story One. I never had one as a kid but do remember seeing them in the Popular Mechanics-type magazines of the time.