These are hardly seen in the US and I couldn't find anything in archived threads. Has Scott stolen, er, researched and verified the listing for these yet? Otherwise, they need to catch up on everyone's military issues, unless they consider them to be like locals. This is a real postage stamp.
Faltpost is fieldpost, military mail. Envelopes or cards with free postage are provided to soldiers in many countries, past and present. Sweden's envelopes have a free return mail stamp, gummed, printed on the envelope back and protected by the large flap. Cut that stamp out and send a reply to the soldier: "No, I can't loan you any more money this month."
The issues date from 1929 to 1963. Facit lists these and most are downright cheap. Facit also lists a fair number of shades and varieties. The above is Facit M11C, a 1960 issue, a really cheap mint stamp.
Mint envelope entires don't carry much of a premium; obviously a lot of these must be given out in recent times. However, these entires have been used by forces overseas like on UN missions, so are valuable for that type usage. I'm sure the reply stamps used that way would have similar value.