https://rbcomponents.com/products/folding-sofa-sleeper-hinge-assembly
This is what FWC uses, I am fairly positive. Maybe not needed, but nice to have extra support base in the aisle. Unless you lower/recess, the E/W side bunk support, you will also, likely need a foot platform to avoid dangling legs. My storage base N/S R/O couch in my Grandby Shell is lower than my driver’s side bunk. So, the couch sits lower to the floor.
The above link is about half the price that Amazon listed it for when I was deciding on my Shell order. I ended up having FWC do R/O couch. Wow did FWC raise the cost of that from 2017 to 2018, ~35%. Glad I ordered in 2017.
Sorry for the 2nd link post, had to walk away, and others replied sooner.