Just to let you guys know, I've sent a message to the fellow on cult movie forum, hopefully we can get some answers on this.
I think this can be the pre-cert tape he is talking about http://cgi.ebay.co.uk/THX-1138-DUVALL-PRE-CODE-VIDEO-VERY-SCARCE-BIG-BOX-/400168142210
Unfortunately, that one ended a month ago.