It could be my imagination but there seems to be some characters
printed on the more lightly coloured rectangle. If you have a
powerful lens with you, see if there's anything to read by
varying the angles of viewing and lighting. Perpendicular is not
always the best.
I use a lens salvaged from a 16mm film projector and have often
succeeded in identifying devices that at first don't seem to have
anything on them. That includes markings that had been rubbed
off, faded with age or burnt away from overheating.