USAF WW2 HS-33 headset

Around 1943 a so-called "low impedance" headset was introduced based on the ANB-H-1 earphone with an impedance of "only" 600 ohms. The proper headset version was designated HS-33. For a period the HS-23 and HS-33 headsets were used in parallel and in order not to confuse one from the other the HS-23 was equipped with a black PL-54 jack and the HS-33 with a red PL-354 jack. It was possible to use the low impedance HS-33 in a high impedance aircraft comms installation by connecting through an MC-385-(*) adapter