I don't see too many precancels for this early U.S. Series. I see even fewer of a precancel that contains a number such as the 7-1 shown here:

I also have a virtually identical stamp without the "7-1" (just the Chicago, Illinois precancel)...I also have others in the set such as the 3 cent (Chicago 6-1) and 5 cent (Chicago 4-1) varieties.
Are these considered common? Was the 7-1, 6-1, 4-1, etc., considered a precursor to the zip codes of today in order to show the post office that may have produced them?