Thankyou wrench1. The most recent I've bought is a British classic, the Astrolite. It has 2 x 300 ohm earphones series wired and an amplified (to carbon level) 300 ohm electromagnetic microphone. It has an approval number label WR721. I've had at least one like this before except that there was no microphone amplifier and there were a couple of resistors wired into the earphone circuit.