Check out this very rare+ (R8) "Smurfs Save the Day" cartridge with the "Kid Vid Voice Module" peripheral eBay Auction -- Item Number: 260695457616
It's never been used, complete with manuals, cable, and all three audio cassettes still in their cellophane. It looks like the day it was manufactured back in 1983, not even smudged with fingerprints.
This is a prize for any collection. Did you know that this device was conceived and invented by Ralph H. Baer, the father of videogames?
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Side note: unused Milton Bradley "Spitfire Attack" 2600, eBay Auction -- Item Number: 260695481729