Crosman 2240, 2250 (but not King Rat) 2260 Stainless Steel Valve, Brass Seal Holder,Stainless Steel bottle piercer, and brass head with seal.  See the main photo.
Made so you can change the seal easily by just unscrewing the front and replacing the seal. For King Rat select it under the options tab. If you order the one for The King Rat it will not have the spike.
Saves money in the long term.

You can add an extra High Quality seal for only £4.00 when you order under the options tab.

The standard brass one has the seal held in place by rolling the front edge over and is not made as a servicable item, so is a throw away, and you have to buy a new complete valve and valve body. With mine you can  change the seal as many times as needed. But they will last a long time.
I have managed to fit one of my new seals to the standard Exhaust Valve but it has to be done by picking out the old one and pushing the new one under the lip to retain it. Being careful not to scratch or damage the smooth face of the seal. I don't like doing this as it seems a bit of a bodge up, which is why I designed this one that unscrews.

I have seen a lot of how to replace the seals on You Tube and do not agree with some of them so will be puting my own version on how to tune and replace the seals on the Crosman 2240/22250/2260 on YouTube soon.