This is a US made NOS JAN spec tube (Joint Army/Navy Spec) made in the late 70's.
They are long lasting, really heavy duty dual triodes, which is great because the are usually used in the driver stage of an amp, aka: Phase inverter, and they take a beating driving the power tube changes (usually were recommend changing the driver stage tube everytime you change power tubes.
Groove Tubes GT-6201-M (code 55085)
Microphone Tube, Very Quiet Selected Version of a 12AT7
£19.96 (inc vat)