OK, a few items:
I worked with CO2 tanks for years at the local soda plant. That looks like a standard 20# tank. I have handled literally thousands of them over the years.
First of all, notice the nut on the back side of the valve. DO NOT FOOL WITH THAT NUT!!
I assure you your life may well depend on it.
Inside that nut is a disc, usually copper. It is calibrated to rupture before the bottle does in case of an overfill or overheat situation.
You will notice some small holes drilled in that nut to vent the gas when the disc ruptures.
With that hose you have on it, you need to be careful. Depending on temperature, you will get somewhere between 1500 and 2000 PSI out that hose when you crack that valve.
And it will be coming out at about -78F. Frostbite can occur if you are not careful. And that much pressure can drive stuff right through your skin like a hypo. Or rip parts off.
I have used hoses like that, but its tricky.
Popoff valves below the regulator........They are set to 120 for a reason. If its used in a Postmix fountain soft drink system, the carbonator, which makes carbonated water takes 90 PSI to work.
There is usually a second set of regulators past that the restrict the pressure to either 40PSI or maybe even 5.
The hoses used in that industry are usually rated at about 200PSI, but some are higher.
If you are going to try to use it for fairly high volume outputs, such as air tools or airing up large tires, any beverage regulators will not work.
They are setup for low flow, and will not pass a lot of volume. Plus they will freeze up if you draw on them hard.
There are companies who make high flow regulators just for your type of use. I'm sure someone here knows where to get them.
As for refilling it, I would not do so. If you have a local beverage company, such as Coca-Cola or Pepsi, you should be able to just drive up and say you need a refill. They sell from dozens to hundreds of bottles of the stuff per week, and are not going to look twice at yours.
They will just grab yours, hand you a full one and charge you for the gas. It looks like it may be made out of aluminum judging by the picture. They all used to be made out of steel, but the light one are slowly taking over.
You probably not be able to specify which you get as a replacement. If yours is aluminum, and they hand you a steel one, it will weight a lot more. If you want the aluminum, you can say something like, " man I hate those heavy ones, you got an aluminum one I could have?"
Like I say, odds are the guy you are dealing with does not care. He is just selling CO2.
Personally, I think I trust the steel ones a little more that the others, but there is no reason to. Its just what I am used to.
I do like handling the light ones. And they will thaw out faster if you draw on them hard and they freeze up.
Couple of other things:
That brass valve is fairly fragile. That cover is more than just a handle. If you knock it over, and it hits a curb or something, that handle will usually protect the valve. Otherwise it may snap off and turn the tank into a rocket.
So, if you mount it in the truck, protect the valve and make darn sure its secure. You do not want it shooting around at 100 mph inside the cab.
Don't forget that there is many many times more volume of CO2 in that tank than there is breathable air in your truck.
If something fails, and its not vented to the outside, you could find yourself trying to breath pure CO2 in a matter of seconds.
And you could pass out from that in less than 30 seconds.
When I say I don't recommend refilling it unless its been inspected, trust me. I have seen exactly one explode, and that was enough!
I may have some other suggestions later.
Meanwhile, here is some other reading.......
https://ck5.com/forums/threads/co2-tales.285389/
