Eyam cover with set of Salvation Army stamps, very unusual! Sold for > £100 on Ebay, January 2018. Covers with single 3d stamp is more common as seen below.
Eyam covers typically sell for high prices because they are rare. Eyam is a tiny village near Sheffield and even the handstamp is rarely seen on anything. Every August they commemorate the plague outbreak that lasted for 14 months and killed almost the whole village. In 1965 was the 300th Anniversary and the “celebration” was formally from 28 August to 4 September. The postmark was probably used before the actual celebration period to let people know in
advance that it was happening. It is possible that it could have been in use on the First Day of the Salvation Army stamps (9 August), but it is very unlikely any First Day Covers were done as none have been discovered by now, over 50 years later. (Reference: personal communication with Brian Reeves, respected UK FDC dealer).