I'd suspect that it was intended for two purposes.  Firstly, to
make less audible the sounds made by some rolls as they pass over the
tracker bar, and secondly, to keep noxious or intrusive objects out of
the spoolbox or trackerbar ports: fingers, liqueur glasses, cigarette
smoke, and whatever else have you.

Mine's missing, unfortunately, so if anyone could provide dimensions
needed for making a replacement, or even better, an original glass
door, I'd appreciate it.

Colin Hinz, the comma-meister.
Toronto, Canada