Parkville, MO, precancel overprint handstamp
The device is called a 'rocking marker' because its printing face follows an arc. During use, the bottom edge is aligned with the center of a row on a postage stamp sheet. Hand pressure is applied, and the marking stamp is rolled towards the top edge. The handstamp is then lifted and withdrawn after this single pass.
The stamps to which this 'precancel' marking were applied are for permit bulk mailing rates, usually by non-profit organizations. Once sold to the organization that would use these stamps, the mail matter was only accepted at the office signified on the stamps (the office at which the bulk mail permit was issued).
This device is "1 by 5," meaning one column and five rows of stamps in that column could be precancelled. Similar devices were made that were "2 by 5" and "4 by 5." Ordinarily, the more frequently bulk rate denominations were used, the larger the handstamp requisitioned by the post office.