[JeffJS]

Hi

Don't worry about the plastic piece as it has no part in any aperture use.
Looking at the small arm you can see that there is a scratch mark, that is from the camera control arm. Probably what is wrong is that the lens arm is slightly bent so that it does not engage the camera. just bend it a little towards the outside and it should work fine.
Another option is to completely remove the plastic sticking up and bending the arm towards the inside, that way the lens will work as the Taks. in Av mode.

Awesome advice bobpur +1! I will remember this.

I had to think about that for a second. With the pin bent like that, the camera mechanism can't activate the lever to hold the lens open. That's fine on a broken lens like the one being discussed here but I wouldn't do it as a matter of practice just to use Av mode with an M/K lens. Still worthwhile knowledge to have though.