Toppop                        

As far as I can tell, this is the only time these (dreadful!) outfits were worn on a TV show.  ABBA were introduced by Ad Visser who 8 years later interviewed Frida

Here's a proper (colour) image of the performance taken from a book and here in black and white.

ABBA were featured in a book about Toppop and a picture from this session was used.