Forceflow,
I recently tried to repair that board on my Buck Rogers and ran into the same questions as you did. It looks like MDF but I am not sure the MDF technology existed at the time the board was manufactured. It seems lighter than MDF (density) but seems to react to water like MDF. In my opinion, it has cellulose/paper fibers in it by the way it reacts to water damages.
If I had to redo the board, I would use light plywood and not MDF (heavier and more messy to work with).
Yves