Just for the record -- there is a barrel-shaped "roller" that goes in a rectangular opening between the top cover and the sliding "clam shell" lens cover. You can fabricate a replacement for the missing stainless steel part using a short length of round brass stock from a hobby shop or the like.

Remove the bottom cover (5 screws) and lift the sliding cover out from the bottom to remove it. Get a piece of brass rod approximately the same width as the rectangular opening and snip off a small piece to fit. Lay it in the opening and replace the sliding cover and bottom -- should be good as new.