Hi Perry,
I'll describe how the upper bunk attaches on my 1974
Boler. You can see what I'm talking about in the upper photo in post #13 of this thread (that's my Boler).
There is no extra board or 2x3. The way it works is that there are two hinges that mount to the top of that lip (the protrusion) and then also to the back of the seat back, up near the top of the seat back. They are a normal "hinge" shape, like the hinges on the interior door of a house.
Because of the way the hinges do their thing, the seat back ends up higher than the lip when you make it into a bunk (you can see this in the photo on post #13 also). Voila!
If you look on the top of the lip (protrusion), perhaps you will see where the hinges were mounted to it. On mine there are three holes at each end of the lip (right and left, out pretty close to the ends of the lip).
On the seat back the holes are harder to see, because they just go into the upholstery without much of a fuss.
Raya