Don't know if this will help, but maybe this will give you some ideas. This is how the gauchos work in our 1966
1The gaucho in the couch position looks much like yours
2. To make the mattress large enough, there is a small bolster (green thing) stowed behind the back rest. It supports the back rest when in couch position and lays down to extend the mattress in bed position.
3. The gaucho cushions have a fabric hinge to keep them together. They also have a 1/4" plywood support inserted into the bottom of the mattress cover.
4. The cushions in bed position.
5. The Walter of Wabash slides are extremely strong. Even though the bed cantilevers into the aisle, it requires no additional supports. Though I made our own slides, they are available from VTS. The slides have stops so that they cannot extend too far.
It's very easy to extend the bed: pull out the base, lay down the bolster, flatten the cushion and that's it. There are no pieces to remove or stow. I don't know if the slides were used in the 50's but I plan to use them in our '55 trailer restoration too since they work so well.
Hope this gives you some ideas.