If you own an Optigan Optical Organ (Crap keyboard made by mattel in the 70s that plays back scratchy sound off acetate discs), then the discs themseves are essential. Without them the keyboard can make no sound.
I've collected many discs over the years (25 or so)... they are rare, but do turn up from time to time. All the discs were made by optigan/mattel, apart from a couple of notable exceptions. One of them, the Songs Of Praise pak, continues to elude me, and pretty much every Optigan owner in the world:
They basically don't exist. I've had it as a 'saved search' on eBay for going on 8 years. Nothing. Nada.
Until last week. The only one i've ever seen turned up on the bay. AND NOW IT'S MINE. I had to pay a ridiculous amount. Far more than i've ever paid for an LP. But it's worth it.
If you want to hear the 'ill' sounds that come on this disc, then visit THIS link
As an aside, I don't know of any owners of a WORKING Optigan in the UK, other than the musicians Damon Albarn, Ed Harcourt and Michael Wookey. They were never produced in the UK, so any that exist over here were imported. They were made out of such flimsy material also that the vast majority of surviving Optigans are broken, and impossible to repair 'cause of obsolete parts. If anyone out there knows of any others... holler