I have long known this set was likely a 1921 issue and not a 1920 issue like the first run of Holsum cards due to the player selection. However, it is also very obvious these Standard Biscuit cards were issued before the E121 Series of 80 as there are players in the set not in E121 (Dave Davenport, Art Fletcher, etc.) and players whose images they changed (Bill Donovan).
Reasons pointing to 1921:
Rabbit Maranville was first w/ Pittsburgh in 1921
Walter Schang was first w/ the Yankees in 1921
Eppa Rixey was first w/ Cincinnati in 1921
etc.
Anyways, it isn't always that you find a card that can totally pinpoint the time when cards were distributed, but I recently purchased this card off ebay w/ no back image, normally I wouldn't be too happy about writing or the obvious trim job but in this case I'll make an exception...
The writing on back says... "Reced this July 5, 1921 [on] the train to Santa [Eva?]"
Anyways, I thought it was pretty cool
-Rhett