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This looks like the handset used with the Ptarmigan communications system. The handset is a little larger than the similar tactical handset used with the Larkspur range of systems and many others. The inserts are the standard dynamic type manufactured originally by STC. Red/White is the transmitter (microphone) and Green/White is the receiver (earphone) There is also a Red/Black which is a carbon mic. The dynamic inserts are nominally 300ohm impedance. These handsets were manufactured by Racal Acoustics, who bought S.G.Brown, the original manufacturers. The plug is not original.