You could argue that Pocket Rockers is a wannabee of the late 1960's Playtape. There were hundreds of titles available in the Playtape format. It is a close cousin of the 8 Track and the technology is similar to Pocket Rockers with an endless tape loop. Check out the excellent Playtape web page by Lynn Fuller  on 8 track Heaven. Playtape items commonly show up on ebay.