Wow! I actually have in my possession one of the schlong books! And looking at the artwork that is rather light and pastel with all the other gracefully thin and "correct looking" arms and hands, the fingers on the backside of the songbook look accurate and thin. But rather than a wrist and thumb toward the inside of the book, it looks like, well, you know what.
Geez, now that it's pointed out to me, it sure looks deliberate.