The printings of HJ and E121 probably overlapped as opposed to one being before the other. There aren't any "exclusive" players to HJ not forund in E121 or any team changes, etc. that one would expect to see. The sets are very similar in their make-up. And as I said before I wouldn't rule out a Thomas being found with the same "C" variation while having an E121 back.
About the Holsum cards, it is pretty concrete that the Type 1's are a 1920 issue and the Type 2's a 1921 issue based on my research.
Type 1:
-Amos Strunk is pictured with the Chicago White Sox, a team he didn't join until mid-1920
-Bob Roth has been found with Washington, a team he didn't start with until 1920.
pictured here is the Type 1 (arms at side) & Type 2 (batting) Strunk cards...
Type 2:
-Eppa rixey is found w/ Cincinnati, a team he began with in 1921
-Walter Schang is found w/ the Yankees, again a team he started with in 1921.
In a lot of these sets there are what I call "anomalies" in that there are players that are shown with teams they hadn't played for for quite some time, the true way to tell the date of a set is to find players that were making their FIRST appearance w/ a team in the year of issue (not the last appearance because that could have just been from outdated info or photos)...
Known anomalies:
Holsum Type 2 (a 1921 issue):
Chester Thomas w/ Boston (last played with them in 1917)
Ray Morgan (last played w/ Washington in 1918)
Harry Covaleskie (last played w/ Detroit in 1918)
Ping Bodie (never actually played w/ Washington-??)
Standard Biscuit (a 1921 issue):
Dave Davenport and J. Carlisle Smith both were done playing w/ the teams they are pictured on in 1919.
