This is a reproduction light guide for the 7909 volume knob.
With age they tend to become fragile and brittle and often break leaving a floppy volume knob.
This reproduction light guide is made in clear resin and looks like the original.
 
Install steps:
Remove your volume knob from the unit by pulling it.
With a hair dryed or heat gun head the knob to soften the glue holding the light guide to the black anodized aluminum part. 
Clean remaining of old glue with a scraper and some alcoohol.
Use a dab of 2-part epoxy glue to install the new light guide onto the black cover.
Remove the metal clip from the old light guide and install it the new light guide paying attention to align the slot.
Install your freshly repaired volume knob and enjoy the green light :)