Jet guys stick together. Say something bad about them and start watching your back!
Someone above mentioned the swab. It makes extra cylinders look like they're growing on trees. Its not really worth very much, objectively, but you won't find one, either. The box is worth a bundle, and they may not even exist out of captivity. The only easy and sure way to obtain a box, 22 cal cylinder, swab, etc is to just buy a package and then either sell off your gun or the parts of the package you don't want.
I'll venture a guess at the value the gun shown by the OP is worth. Maybe $800 on the low side and $1200 on the high. If it were complete, as in all the accessories, cylinder, inserts, box, etc it might bring $1500 around here. I have seen them sell for more, but those are rare.
No one much seeks the 4" guns because they make even more noise. Yeah, I'd take one if I saw it reasonable enough. The 8 3/8s guns are kind of long and unwieldy.
In all fairness, there's no real reason to use the stupid inserts. They just add in a level of complication that isn't needed. You'd be better off with a K22. Same goes for the extra cylinder. Just buy and shoot a K22.
Jet ammo is around. At the last few OGCA shows I've been seeing Jet ammo in the Green boxes for $50. Was a time when you never saw any. I'm no longer in the market for any. When I bought my current Jet it came with 10 full boxes of ammo. It and the gun cost me $700.

Since that time I've accumulated some half empty boxes and a bunch of fired cases, and some "never" fired ones in the plastic bags.
Correct bullets are scarce, but available. Dave Keith has developed a system of taking .223 or .224 bullets and swaging them down to .222 in a die he's made for the purpose. He enlists his grandson to do that job, paying him handsomely. He was generous to give me a box of his finest and I've got them next in line to load. I just assume they'll work fine. Now I'm feeling badly for not loading and testing them.