Okay, what is this greasy felt thing in my door?

Hi All: Question of the day. I was just cleaning and lubricating the manual crank window mechanism on the driver's door of my 74 Duster. On the top of the slider that goes up and down a circular rail to lift and lower the driver's window, there was an icky felt - like circle that was completely encased in window grease. It slid up and down on top of the slider mechanism and rode up and down the circular rail. Has anyone seen one of these in their car? What does it do? Is it supposed to be there ( I took it out)? Should I replace it? With what? Thanks! BOB

IMG_1101.JPG