If I can find it in the pile of stuff, it was the pair of carts that let you control a laser light show from the Commodore 64. I showed it to Jeri Ellsworth once, but I bemoaned the fact that it would cost nearly $1,000 to get the proper lasers and mechanisms. Engineer Dale Luck said that he would help me out in scrounging those parts, but I never got any farther in the project.
Fresno Commodore User Group http://videocam.net.au/fcug
July 25-26 Commodore Vegas Expo - http://www.commodore.ca/forum
and click on ComVEX