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.
Original description from the Stone Company: Merlin is a chestnut pinto Standing Drafter designed by Lisa Bickford. There is a Non-Custom version and a Custom version. The Custom version has a loose mane and tail, and a twitched ear. This run was available in matte finish. The total run of both custom and non-customs was 250.
Update: There were only a total of 100 models made in either version (about 50 with bobs and 50 without). Only two were made with no bobs and a twitched ear. There were 3 glossy models made, all went to the artist.