Peter, Michel Automatenmarken-Spezial-Katalog does not list this item. I think it would be classified as a meter. By definition, an ATM label/stamp must be available to the individual customer through a machine (typically in a post office lobby, airport, or mall). A meter is applied by a postal employee to a specific package or letter and is always dated at the time of use.
If you have one of these PT Post labels on a small cover, that would be a nice item.
Here's a Swiss meter cover I found in the dollar box at a stamp show.
