When is a "Pocketbook" not a book?

Here's another one from Ken Johnson. Future Science Fiction #28 (which I believe came out in 1955) is a full-sized magazine issued by Columbia, but the red symbol to the right calls it "A DOUBLE-ACTION POCKETBOOK." Ken, who is an expert in Sci Fi magazines, says it is the only time this appeared on a mag.

Scan by Kenneth R. Johnson