Hi Charles,  If you try to wet anything, it should be the concave side
of the board, then put some weight on the convex side.  Or you could
try to steam it and then drive your car on it.  In either case, you
need to remove the finish first.

But I doubt if you'll have much success.  [The photographs show that
the] board is not just warped 'a little' -- it's completely twisted and
destroyed.  The back edge is straight and the front edge is curved.
Even if you get it fairly straight, it will never be right (straight).
If it were me, I'd just remove the bent board and make a new one out
of similar material.

By the way, are you sure it's original?  I have a real Regina disc
cabinet (with a Regina label) and the separators are an integral part
of the cabinet.  I have another cabinet that has a slip-out disc case
like yours, and it's clearly a piece of furniture that has been
re-purposed.

Regards,
Craig Smith
Upstate New York