2 PCS Vintage Foster Microphone Inserts New Old Stock Rare Japanese Made Mics x2

Had a box of these new old stock mic inserts sitting around for years. I am not sure whether they are for telephones or 2 way radios or vintage studio microphones.

New old stock, never used.

Foster Part number 9601D

35 mm Diameter.

18 mm Wide.

2 Screw terminals. 

Made in Japan by Foster Microphones.

Pictures make up the description.

Thanks for looking.