it looks like a Raytheon knob ..... I actually might have some at work .... my Company use to deal with Raytheon parts a lot during the late 80's into the 90's ...... there's a stash of parts still left in the older work area ... I've seen knobs like that in there ... almost positive .
you can just search skirted plastic knob .... you'll see a crap load that come close .... you might get lucky .
if I find one on Monday .... I'll get it out to you