i have no doubt, and have thought for a long time, that stuff like this is just the tip of the iceberg. the last 4 years have been huge for uncovering new stuff, particularly video footage, but some great audio too (this is a perfect example). when i think back 4 or 5 years, there was next to ZERO video footage in circulation.
recordings like this must be everywhere. in peoples homes, in boxes, under staircases, on hard drives - everywhere. CC traders have always been a bit fickle when it comes to sharing. thank god that's changed a bit now. sites like this, and recordings like this, do a LOT to change that.
the only way to continually uneart stuff is to keep digging. and the only way to make sure shows go from "rare" to "in circulation" is to share everything you find.