As has been stated, these are "reply" stamps. "Det Bästa" is the Swedish edition of Readers Digest magazine -- DB/RD have always been marketing innovators. This was a fairly long-lived experiment / practice. The idea was that advertising mail respondents would be more likely to respond if there was a "stamp" on the reply envelope/card and/or if they had to put the "stamp" on the envelope/card. This indeed worked to increase response to advertising mail solicitations.
The system basically worked identically to typical "business reply mail". These were just labels in a sense; the post office did not much seem to care if they were affixed or not. At the post office of delivery, accounting was done and the recipient company charged for the incoming mail.
The stamps had no actual financial value; the company creating them simply obtained the license/permit from the post office (just like business reply mail) and printed "stamps" that met postal guidelines.
Det Bästa was the primary user of these. However, several other Swedish companies had one or two issues each and a few Norwegian companies had one or two each as well.
Yes, they have been listed by Facit from time to time in simplified form, but not recently.
AnthonyUK mentioned the primary catalog/handbook that lists these. I do have this catalog in stock ($14 plus shipping).
http://www.jaysmith.com/Literature/...html#a_19014I also have a range of these stamps in stock (but not on my website). I also have a range of them properly used on covers. For the most part, the individual stamps cost 25 or 50 cents each, etc., though there are a few that get up to $5-10 or even a little more. The covers are typically a couple/few dollars each and sometimes are found in groups of 10 or 20 all-different because that was how a major Swedish dealer (Gummesson) sold them back in the day.
I welcome requests for them.