A: That unlabeled potentiometer is a vestigial item from the LA-2A’s origins as a broadcast compressor. It controls sidechain pre-emphasis, providing up to 17dB of boost at 15kHz. It was intended to control high frequencies to prevent overmodulating radio transmitters with a hot signal. In other words, it makes the compressor more sensitive to high frequencies, similar to how a de-esser works.
The control is “flat” when turned fully clockwise. As you turn the potentiometer counter-clockwise, the compressor becomes more and more sensitive to high frequencies.