Wow, that was easy.
I've been looking for a fix for a couple months but I guess I hadn't found the right combination of words to search for. Thanks!
It is working much better now (even though there's no real obvious change in the height of the screw, it's just scratched up a bit around it)... but one thing I do notice is that if I twist it slightly, the max aperture changes between F2.4 and F2.8. The correct max is F2.5 - which I occasionally see very briefly when it's jumping between the two settings.
I don't know exactly what the mount is trying to tell the camera - I wouldn't be surprised if it's not relaying the exact correct information - but it's strange. FWIW, on the K100D, it showed F2.8 as the max (I haven't tried mounting it back on there yet.) From a few quick test shots, I don't see much different in DoF between F2.8, F2.5, and F2.4. The latter are slightly darker than the F2.8 which would make me think that it's not changing the aperture any, just using a faster shutter speed. Strangely, F2.4 seems a tad darker than F2.5 even though the shutter speed and ISO are identical. Strange indeed!