Filter, Radio, FL-30, USAAF

Genuine FL-30 radio filter, intended for permitting ground-to-plane voice transmission simultaneously with the steady tone from the radio range. Manufactured by The George W. Borg Corp. The FL-30 was an evolution of the FL-8.

Excellent condition. 

http://www.radiomanual.info/schemi/Surplus_Handbooks/Airborne_radio_equipment_handbook_1943.pdf