The Signature Series was a program where an artist would design a model, and then the Stone Co employees painted them. Each model had a stylized sticker of the artists' signature, and came with a Certificate of Ownership.
Enigma is an appaloosa Weanling designed by Laurie Jo Jensen. You could only purchase this weanling if you ordered at least three Signature Series models at the same time. If you ordered five or more, Enigma was free.