This is an unused “verified reception” stamp from KSEI radio in Pocatello. During the 1920s and early 1930s it became a fad to collect these stamps from stations far away. Some collectors had stamp books in which they kept their prizes.
Listeners who heard a weak signal from a far-away station sent that station a card telling the details of what they heard, such as the subject of a newscast, what a commercial said, or what song was playing at the time. The stations would send back a card with a stamp on it verifying that the description matched their programing at that moment.
Some of the companies producing the stamps also sold them unused to collectors. This is one of those unused stamps, which I am donating to the History of Idaho Broadcasting Foundation for their museum in Caldwell, which is housed in the old KFXD studios there. It isn’t yet open to the public, but we’ll keep you posted.
The reason this odd hobby existed is that atmospheric conditions at night make it possible for AM signals to travel great distances. The AM waves bounce off the ionosphere when darkness falls. If the receiver is also in the dark, the signals can be heard halfway across the globe.
Most AM stations don’t bother printing up stamps anymore, but the hobby still exists among shortwave enthusiasts. It’s called DXing. The D is for distance and the X is for unknown. There are magazines about it and online chat groups. DXers get all the relevant information to prove they heard the station, then send the station an email for confirmation.
Back in the late 60s, when I was working at KBLI in Blackfoot, I remember getting a DX card from someone in Japan who had heard the sign-off recording just as the sun was going down, the perfect time to catch a signal from a daytime only station.