I came across this on Wikipedia, it made me laugh at how random it was:
In January 1981, Ulvaeus married Lena Källersjö, and manager Stig Anderson celebrated his 50th birthday with a party. For this occasion, ABBA recorded the track "Hovas Vittne" (a pun on the Swedish name for Jehovah's Witness and Anderson's birthplace, Hova) as a tribute to him, and released it only on 200 red vinyl copies, to be distributed to the guests attending the party. This single has become a sought-after collectible.
https://en.wikipedia.org/wiki/ABBA#1981.E2.80.9382:_Final_album_and_performances