[BILL]

There where coin operated radios for Motel/Hotel use. The shape of it I would say that it is a PHILCO,
This shaped knob was used originally on Philco house radios around 1930 they where light brown , later on these where used on the Transitone automobile radios up to the mid 30's mostly for tone controls found on the speaker and radio boxes, and some early head controls, they where all black .
If I where to guess on auto radio, I'd say that these where for a set that the on/off/ volume control lock where all one piece, Ive been working many years on vintage auto radios ,many early 30's with locks but with no key like this, If you find out what it is ,please let me know, BILL