I'm also waiting for Atlas to rerun their Hart ballast cars in the ATSF scheme without reflective emblems, and hopefully with "ATSF" instead of "A.T.S.F."
My Santa Fe Painting and Lettering Guide (Hendrickson) says that the round reflective emblems were used from 1958 till 1971 when they were replaced by white reflective rectangles. It appears that they were applied to new cars, and cars that were completely repainted, but not necessarily to other cars, including those that had repair work done. For my 1960 setting, I'm using cars with "A.T.S.F." with the excuse that they haven't been repainted for a long time.
Regards,
Ron