[ARC5] Weird headphones.

[Kenneth G. Gordon]

Hello again, Gents:

I have been using a set of headphones for the past several years that 
have always intrigued me.

The outsides are a thin bakelite, tear-drop shaped housing with nice semi-
hard rubber pads fitted over them.

Inside of each ear-piece is an element with the following marks on them:

ANB-H-1A, then in paint PRD1

The headband is engraved HB-7, appears to be made of stainless steel, and 
has leather coverings over the two adjustable spring "loops".

When I found them in my junque, they were missing the cords, so I wired 
them in series, properly phasing them in the process.

They have always worked well for me, although I do have to replace the 
cords periodically since the wire I use is thin and easily broken.

My questions are 1) what is the likely impedance of these (I have assumed 
600 ohms), 2) doesn't the PRD1 indicate that they were probably used with 
either a radiation meter of some sort or perhaps a mine-detector. 3) how 
old might they be?

I have never seen a set of military headphones with tear-drop shaped 
housings before.

Ken W7EKB