The fake diamond stacks appeared on 268 for the 1949 Chicago World's Fair, then on 268, 319, and 345 for the filming of Denver & Rio Grande in 1951. 473 was painted in the Bumblebee scheme in 1950 and got the fake diamond stack. Neither the scheme nor the stack lasted long on 473. After it derailed and took a partial swim in the Animas River in 1951, 473 was rebuilt with a standard paint scheme and standard straight stack. 476 and 478 got their fake diamond stacks in the 1955-1957 timeframe and wore them essentially until the end of D&RGW ownership of the locomotives. Oddly, 473 did not get a fake diamond stack again until 1961 or 1962. The fake diamond stacks were a sheet metal affair that simply slipped over the standard straight stack.
As an aside, 1950 saw a very late start to winter, especially on the east side of Cumbres. As often happens, winter hit with a vengeance when it did come, and the last runs of the San Juan in January 1951 were run in heavy snow and record cold.