It appears that it's a device to regulate the current on the contacts to increase/decrease speed. As it rolls against the contacts it increases/decreases the current.
It's a type of potentiometer. You have to look for the little holes the spring legs might go into and assemble the cover. Manipulate the contacts to see the type of motion that it allows. It might be a case where you have to have another hand to place the part in place and reassemble the cover at the same time. I have many old kitchen aid mixers. This doesn't look reliable to maintain the correct speed especially under load. I'm not gloating but I'm interested in the nefarious activity of these folks. Good Pic.