Santa Fe introduced the warbonnet version of its blue and yellow freight scheme in June 1972 on SD45 5524 and U28CG 7904, but various details of the scheme were "tweaked" as units were repainted over the next few months before the final version of the scheme was adopted. Bob Finan found SD45 5516 at San Bernardino, CA in December 1973 modeling an intermediate variation. The uncomfortable placement of the billboard Santa Fe lettering on 5524 had reverted to the better-balanced location used on the old blue and yellow pinstripe scheme, and the noseband stripes had been extended all the way back to the cab. The shape of the warbonnet S-curve was evolving in the right direction, but hadn't quite made it yet. In the final version of the scheme, the bottom part of the curve was a bit taller, such that the duct running from behind the cab to above the rear truck was blue instead of yellow as on 5516. The 5516 also has yellow stepwells and black trucks, which became blue and silver respectively in the final scheme.