Even though the first image in this thread is 6 years old, I can't resist posting a reply. Bob Payne wrote about these is his 2 volume "An Exhibit of U.S. Classic Machine Cancels, 1871-1991" as cancels from M-36 facer cancelers of the National Cash Register Corp, known 1966-1974 with 31 initial combinations to identify the station and machine. (While many of these covers are on airmail, this is due to the nature of the source of the covers than any indication of sorting by class.)


Second, the current posting, this is the inkjet / spray cancel commonly used on flats-sized mail in the U.S.
P&DC = Processing and Distribution Center
