Don't the sidewalls just slide in and out on tracks?
I have a '76 Royal, a '76 Mesa, and (for parts) a '76 Ramada, so I am well familiar with the system you are describing.
If you were to take the system apart, you will notice that there is a groove in the bottom of the side wall (plastic channel) and a corresponding groove along the top of the metal bed end frame. The living hinge sits in between these two. The bed end portion will slide along the hinge as you move it to put it into place or to take down.
The old (original) material was made of plastic as well, which may give the mistaken impression that it is a tongue-and-groove or track system. Greg and Colleen (Redwood) make an absolutely awesome replacement product, or you could always order it through Apache Replacement Parts - though I STRONGLY RECOMMEND you go with Greg on this one.

I will update this with pictures later today if possible...