Usually when I see those kinds of knurled knobs, one is echo (volume presence and delay), the slide pot is something unique you don't always see an exposed control - maybe if it was from a home Equalizer control - which this looks to be a slide rheostat from some stereo equipment.
Interestingly they were willing to invest into the effort to have to cut a notch or slot, or even make more holes by drill, meant the control was specific and required for their needs.
It is also "backset" so the control couldn't be mistaken.
The Slider control cannot handle a lot of power they rated for about 1/2-watt composition - so the exposure to the open elements tends to diminish the sliders uniqueness because it can attract dirt and moisture ruining the sliders wiper contacts and their ability to slide effectively without injecting a ton of noise.
They are also not shielded very well.
If you're expecting pristine cover and front panel, they took care of the until - for it does not have much in scratches or wear scuffs.
The radio will need help being brought back to OEM because, like any used car or truck, they operated and added as the previous owners needed to obtain a level of performance
- unless you like to see that is inside the motivated minds of whom messed with this radio and now, they're giving it up
- usually signals either the family member passed on and doesn't need the radio with them to wherever they went
- or the family is looking to make a quick buck off the enterprise situation and take advantage of the demand the market will bear.
Possibly one of two things - Front Panel Dimmer (like Frequency counter and or entire front panel display) - or using the slider position - some form of sound to carrier level / frequency control - would have to "look inside" to remove all doubt.
IT's an extra set of controls - for something(s) that have been added to the radio - to me, it would not be a substitute for any front panel knobs, just more knobs for the effort to control something inside
I just came across something...
These older Echo mikes that were popular in the 90's the ECB4+ from Diesel/Workman/Superstar
They also had that slider control in them. So, I would think this is just more of the effort to install the Echo mike permanently.
