WAGNER / TRISTAN / SAWALLISCH / BAYREUTH 1957

Hello,
can anybody provide any information about the above recording? I think
of sound quality, orchestral performance, performance of the
conductor, and especially the quality of the protagonists' voices. How
are Nilsson/Windgassen here in comparison to the five resp. nine years
younger recordings under Böhm (Bayreuth 1962/1966)? I already have the
Böhm 1966 DGG set. Should I go for the earlier? And how? I know the
Sawallisch one was once available at Melodram, or imported from the
USA. Does anybody know whether I can still find it in Germany or
through the web?
Thanks a lot, and season's greetings from Mainz,
Karsten