In process of replacing window felts (inner and outer) on both doors and 1/4 panels. Always easier to do with glass out of way. Decided to remove the window regulators and clean them up. Found the strangest thing. The two door regulator channels that are fixed to the inner door had what appear to be "thin cardboard strips" inside the the channel. Not sure why they are there. Possibly to keep the regulator rollers tighter in the channel !!! Has anyone ever seen this before? I don't see anything listed in any old GM parts book nor do these strips show in any illustrations. Have had this car for over 50 plus years. First time taking regulators out. Curious.