I think it was actually a twin Andy, created by using 2 Greeves Silversone top ends, and I think Bob Stevenson made the cases.
There was a discussion about it a while back, over on the motorcycle racing nostalgia forum, and I think someone posted a photo, if I can can find it I'll post the link.
Edit, Not so sure now as the Greeves forum talks about the engine being a 500 single unit designed by Greeves for motor cross!!