This Leica Summarit-M 1:2.5 / 75 has a stuck aperture ring because the lock ring, that holds the aperture ring in place has come off.
It's "easy" to fix but you need a tube tool to unscrew the retaining ring, I have made mine from a lamp amarture, grind 2 nothes so they fit perfect in the retaining ring.
When take out the lens assembly, take care of the little steele ball and spring..
To make the lock ring fit more tight, bend it gently so it get's smaller in diameter.
When assemble the apere ring, set the aperture to something in the middle, it will make it more easy to assemble. When the aperture ring sit proper keep holding it when put on the lock ring so it sit correct.
Then assemble the rest
