Same answer; different angle of attack. I see a couple questions that are not in alignment, let alone the antenna
Diddo everyone, I'm just adding annoying information and history.
I'm going to attack this like a chef... reverse engineering lol
You asked "What is it?". It IS a potentiometer. But that answer might get me smacked. Then you ask if it is an internal antenna tuner. Nope, it's just a meter adjuster. It will not change anything outside your radio. It's a meter calibrator.
Purpose: Why did they put an SWR meter inside some CB's? Well, "Back in the day" many CBers would only have one CB because the damned things cost $300-500 in 1970 dollars! They would have a slide bracket to transfer the radio from the house base set up to the motor home or to the car. Another common transfer was truckers that worked for a company like Roadway or Yellow Freight and never got the same truck twice. Each day was a different assigned tractor. That is where the vice-grip antennas came from. The truck shop welded a bar over the door to mount the quick connect antenna. They had to check the SWR in a quick fashion and get on their way. Remember that it's a sloppier way to check the SWR and is only to be considered a ballpark figure. You should always have you SWR checked with a real meter when installing on your truck or car. Since I take it you are a trucker, you would never need to use it once you set your SWR... Until you drove down a logging road in the Arkansas woods and tore the antenna off the truck (personal experience LOL). Your in-radio meter would allow you to get a ball park setting until you could get to a radio shop and have your new antenna fine tuned.
For those other newbies, lets get technical...
An SWR (Standing Wave Ratio) calibration knob sets the "zero point" for the reading you are going to take on 2 or 3 different channels. You notice the Modulation or "S" scale meters start at 0 on the left, and go up to the right. For the SWR, the zero is at the right end of the meter because you are going to measure the watts going out. Some of the wattage gets reflected back into the radio, wasting energy for the most part. In the old days, this extra frequency coming back into the radio would overload the crystals and blow up the final amps and make a general mess, killing your radio dead. Today we have IC's that do not blow up, but any wattage not going off the antenna and getting reflected back into the radio means it's not going toward your intended target; the other CBer. Thus reducing your 10 mile coverage to 6 or 4 miles in a heartbeat. People complaining about only getting out 1 or 2 miles usually have an SWR reading of 3:1 or higher. It's like having a 400 HP engine but the transmission is going out.
To test SWR, flip the switch to calibrate and key the mic. Calibrate the needle on the meter to the set point near the right side of the meter. Usually marked by the word "set" and a line or thick block in the red line. Then flip the switch to SWR and key the mic. The needle will give you a reading. 1:1, 1.5:1 2:1 etc.
Next: How do you reduce the SWR (or reflective signal)? By making the antenna shorter or longer. The lower the frequency you transmit on, the longer the antenna must be; the higher the freq, the shorter the antenna. In CB (11 meter) the length is 102", as in a 102 whip. Here's a cheesy example. Lets look at just a straight antenna with out the coils. Have you ever looked on a cop car? The straight little stick they call a spike antenna. Well the 10 inch or so ones are VHF (140-150ish) and the little 4 inch ones are UHF (450-470ish). So you are going to adjust the antenna to the lowest SWR reading. A "flat match" of 1.0:1 is desired but not always achievable. If you do a check on ch. 1 & 40 you will note one is higher than the other. This is where the shorter and longer comes in. Follow the antenna tuning instructions to get the best possible match.
Next: What gets matched to what? Some people think the radio gets matched to the antenna, or the antenna to the radio. Nope. The antenna gets matched to the ground plane it is sitting on. Nothing else gets matched. Once you match an antenna to the vehicle, you can put any radio on the end of the coax you want. I'm sure some antenna junkie is going to tear me apart on this one, but I'm just trying to make a Novice Point for the new guys to CB. Another thing to remember is that if you have a magnet mount on the roof of your pick up and you move that mag to a different spot on the roof (say to haul some lumber), you just changed your match and need to put it back a.s.a.p. On a truck mirror mount, opening the door changes the match so you must check the match with the door closed. Cars alike, need to be tested with the doors closed. So, moving ANY metal around changes the match, because you just shifted the ground plane.
Happy Tuning!
