[Drake] SP-75 Question

[Ed Parish K1EP]

I was examining an SP-75 today and had a question that I couldn't readily answer.  On the bottom of the chassis, there is a panel that is attached with four screws.  It is a little smaller than the chassis, but not by much.  One would assume that it would give access to the bottom of the circuit board, and it does.  But there are no apparent adjustments that one needs to do, I think.  What is the purpose of this panel?