The original Model 21 had reinforcement bosses on the sides of the action. Subsequent usage showed that they did not contribute to the strenght of the action and for cosmetic/weight reasons they were ground, filed off leaving a flat surface that some thought/think more attractive for engraving and save a bit of weight. The 'flat side' option then became synonymous with the 'Custom Built' guns and later the Custom Shop guns.
Double triggers were orignally offered while the Winchester single triger was in development. At any time during the manufacturing history, one could and still can from CSMC order a Model 21 with double triggers.-Dick