ABBA appeared on the daily programme "Drehscheibe" and performed "So Long".
The clip was obviously filmed on the same day as the "Disco" appearance recorded on 30th January as they performed in the same studio and looked very much the same. The differences were: the studio was much darker; there was no audience; and the girls wore their feather capes.
ZDF only kept performances that were pre-recorded and they don't have this one which leads us to assume that the performance must have been LIVE.
Thanks a million to Guido for all of this information. Extra thanks to Jozsef