The material used in 1939 for knobs and and other parts was not plastic. As I understand it it was a cellulose based material similar in composition to movie film and film negatives prior to WWII.  As 39 plastic deteriorates there is an acetic acid smell. I think 40-41 plastic is a different composition because as it deteriorates it smells like puke. The main culprits in causing plastic deterioration are elevated humidity and temperature. I've had good luck bringing back 39 plastic if