We seem to get two good years from a set of checkvalve jet extenders, probably could get much much more but the occasional scratched brass body or chem residue makes most guys just put in a new set and be done with the guessing.
Everyone above is correct, it's just a ball, a tapered spring , and brass Allen head "keeper" that holds them in, I will emphasize they are made of brass and on purpose, with a nickle plate, it's the brass between two stainless parts, the jet and the jet manifold, so you will never strip a thread at a bad time. As a final note, the SS jets do seem to do BETTER with a double wrap of teflon tape so you don't tend to bottom the jet out in the checkvalve, you SHOULD still see threads on the SS jet when it's snug.