Actually, it's screwed-on. You have to remove that cap, the nickel thickness top portion where the seam is. Careful use of a knife or razor blade should pop it free.

See if a careful knife or razor blade might pop it off, the spring clamps could be pretty strong. If that fails to widen the gap, then it's a screw-on cap. You'll need to treat it like a faucet aerator that you don't want to scratch, by wrapping the cap with cloth, leather, rubber, plastic or even liberal tape & take big (long) pliers to it. It should be normal threads, loosen to the right.

Otherwise, I might try the shoulder sleeve below the stem...that looks removable. Not that I've seen it before, but the cap could be screwed from the inside as that stem looks quite thick.