Just received these in this afternoon's mail. The sender was the agency that oversees Medicare claims for my Mom. The indicia is from the same meter as noted on the bottom meter number ... so they are quite common:

A curious thing when studying these two examples: The number below the ZIP Code one would assume would be a count number as to the number of pieces that went through the meter, but in this case the first one (#306) is dated December 29, 2011 and the second one (#307) is dated December 30, 2011.