Pictures/diagrams of cabover sliding bed details?

[Detroit80]

Recently picked up a 1987 Fiesta on a Ford E350 chassis. One of the previous owners completely pulled out the bed over the cab area, and replaced it with just a short piece of plywood. Absolutely nothing left of the original sliding bed structure.

I'd like to rebuild it, but not sure how to do it. Curious if anyone has any decent diagrams or pictures of the structure, or even something similar to it? I've been looking at a bunch of videos of people with similar vans, but all they ever do mention sliding bed, slide it out a couple inches and push it back in, and say they only use it for storage anyways. Never enough to get an idea of how it works to make one long flat platform after pulling it out

Or is really just as simple as one sheet of ply sliding under the other, and there's always a "step" between the two when extended?