The ERP database seems to indicate that the large head die was in use at the beginning of die 9's run, as I didn't see any small heads on database entries on 1929 paper (with the caveat that not all images in the database had the resolution to do a proper identification).
This non-scientific survey would seem to bear out Bill's position that one die was early and the other was later.
It still remains to be shown that the small head die exists as a worn version of the original large head die. I'm of the opinion that it is not. But, the whole process of going from master die to hob die to working die is complicated enough to yield just about any verdict.
Looks like just another vinyl working die variety of die 1. The vinyl plates wore quickly and produced an untold number of unlisted (deservedly so) varieties. A few of these varieties were mistakenly listed at one time but have since been deleted. See my book: Collector's Guide to (the) Circular Dies, the Washington-Franklins of U.S. Postal Stationery
Ah, thanks! I was more confused about the way it looked than the die number I guess. Have a fair amount of these, and just started to organize them. This was one of those "one of these is not like the others" cut square. It's just... large I guess on the lettering. Hadn't come across any others, but thanks for letting me know it's common!