Mullard M8137 / ECC83 / CV4004 / 12AX7 NOS Made in Great BritainDated 1964, box plate version with the typical wrinkled glass used with early Mullard preamp valves.The M8137 was Mullard's highest spec ECC83 type tube made for the british military purpose. the codes:64164 = 8137 / CV40041 = first version R4F1 - For exampleR = Radio Valve Co. Mitcham Factory4 date code 1964F June1 1st week
