Never heard of this one! My guess is that it was a handout item at one of the World's Fairs in the Sixties. One year (I believe it was '62) the handout was (or included) some unique decals. I've got a couple sets of the decals, but I've never heard of, or seen, the key.
I couldn't guess at the time frame, but if I had to, I'd say it was done after Budd Anderson left AMT in mid-'64. After his departure AMT did run some comic book ads with a cartoon "Kat from AMT" that resembled the key. But there were little figures included in AMT kits that resembled the "kat" before that, so who knows. Nice find!