I had one, in a "Bloom County" cartoon book[1]. It was a flexible black record the thickness of a thick sheet of paper. It even had a spot to place a coin because the black plastic was thin and slippery. It played pretty well though and had two sides.
Someone is selling one on amazon with a decent image.
edit: see the comment with the art article about "Flexi-disc"
As an interesting aside in the "alternative record formats", when as a child I had the "mickey ghost chasers" book. It came with a little hand held box, which you'd put over mini "records" imbeded in the books[2]. You 'd hit a button and it would play audio. The underside was like a weird mini-upsidedown record player
Someone is selling one on amazon with a decent image.
edit: see the comment with the art article about "Flexi-disc"
[1]https://www.amazon.com/Billy-Boingers-Bootleg-Dalton/dp/5551...
As an interesting aside in the "alternative record formats", when as a child I had the "mickey ghost chasers" book. It came with a little hand held box, which you'd put over mini "records" imbeded in the books[2]. You 'd hit a button and it would play audio. The underside was like a weird mini-upsidedown record player
[2]http://www.thisoldtoy.com/l_fp_set/toy-pages/800-899/800-tal...
again the best images I found are on amazon (see the orange circles on the page.
https://www.amazon.com/Disneys-Ghost-Chasers-Fisher-Price-Ta...