I have been search the internet for some time now trying to find more information about the Riksluftskyddsförbundet or Sweden Royal Air Defense Society aka Sweden National Air Protection League.
Language has been the major barrier.
RLFS - Riks Luftskydds Förbundet
If translated individually reads
Nationwide Protective Air League
But as a statement becomes Riksluftskyddsförbundet which can be translated as "Royal Air Defense Society" or "National Air Protection League."
I have found to more examples:


Apparently the society was founded about 1937 in response to the "German Threat" and lasted until 1945.
Charity stamps or seals were sold to help provide funds for the society.
I believe your stamp was the first issued about 1939.
The two stamps I posted are probably from 1940 to 1944.
Hopefully there are some Swedish Experts out there to offer more information.