Ah, that explains it. By the early 90s I'd more or less stopped listening to music all together and what little I did listen too was no later than baroque. (I didn't really start again until I fell into bandom in 2007.)
I seem to remember going to at least two but while I can pinpoint the first because of the venue I simply can't recall where the second one was. I suspect it must have been the 1978 tour though.