Yes, this is the last song (No.14) of the Eltham Well Hall concert in 1972. The concert is still seeded as a "lossless" torrent at "dimeadozen.org" and is seeded nearly every day.

There's another a capella number on it called "Reynard The Fox" which is kind of similar to "Rigs Of Time"

Since the sound quality doesn't necessarily require a lossless copy, the MP3 files of the concert are also available in a zip archive at:

http://tinyurl.com/2g22w36

If anyone is interested in a lossless copy, you log in as a member to "dimeadozen.org" (free membership) and click the following link:

http://www.dimeadozen.org/torrents-details.php?id=113271

It's a completely legal torrent.