I think the 135 2.8 (and maybe the 3.5) are the only lenses Olympus made this way. There is a plastic insert inside the lens mount. The spring that pushes on the lock button has two holes in it. This spring is put down on 2 plastic studs, the studs are melted down over the spring. With age, the melted plastic part breaks off, spring pops off, lens lock button doesn't work. I remove plastic insert, drill 2 holes in it, install 2 small screws. John.