[Boatanchors] 75A-4 Reduction Knob Problem

[Richard W. Solomon]

A friend gave me a 75A-4 to work on and one problem I see is in the
4:1 Reduction Knob. You can tune one way, but when you reverse the
tuning it takes a few turns of the knob before it engages the tuning
shaft. 
Is it safe to assume the one small screw opening I see on the knob is
for removal ? Never having worked on one of these I will need 
guidance before proceeding.

Thanks for any help,

73, Dick, W1KSZ