Yes, I too was very confused about the AMT/SMP connection. When I was doing research for the CorvettePromo.com website, a noted promo historian emailed this information to me. Here's what he wrote:
"The AMT/SMP connection can get rather complicated. Technically, they were two separate companies, but more in name only (most price lists I have from the time list both AMT and SMP kits, under the AMT banner). In 1980 I received a letter from Erik Erickson (Director of Operations for Lesney AMT Corp. at the time). Eric started SMP (Scale Model Products), "as a supplier to G.M. Photographic to fill their needs in their dealer promotion in 1956." SMP was a subsidiary of Detroit Plastic Products in Mt. Clemens, Michigan. SMP was sold to AMT, effective July 18, 1961. Trademark and all tooling sold for $225,000 ($100,000 in cash and $125,000 in AMT Common Stock). It is strange, but until they were written up in an issue of Model Car Journal, I had never seen a SMP kit for the 1962 annuals. Those 1962 SMP annuals did exist. As far as I know, the AMT and SMP kits were produced side by side. I know before the boxes were printed with the car shown on it, some SMP kits came in AMT boxes. Interesting, the first SMP kits came out in 1957. These were just unassembled promos (molded in acetate plastic) in a promo-size box, but made for rack display. No Corvettes were offered, just hardtop and convertible versions of the Pontiac and Chevy. I wrote an article on them for the December 1980 issue of Car Exchange."