Peter: Mine has the same opening, and once the felt lining was removed, the opening was covered by a piece of rubber.
My thought (pure speculation) is that the opening was designed to hold a gang plug and the two square holes on each side were part of the gang plug where fixed bolts would come through for nuts to be attached without spinning the bolts while tightening. The gang plug would be something similar to an electronic module or a bank of resistors. Again, pure speculation.
Related posts indicate the opening was not used by the factory.
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