I didn't know that Kellogs Frosties issued a snap-together SR-71 Blackbird. They did though!
In 1976 they released a free kit for Frosties eaters and you can see it on the cool site Cereal Offers
Its just one of countless freebies from Frosties and the site displays them all!
Did you ever get the SR-71 gift or any other cereal toys?