The M8083 is a special quality version of the EF91. The M8083 has a more rugged build than the EF91 and was designed for applications where vibration during operation was unavoidable and where the performance could be predicted when subjected to such vibration. The obvious application is mobile communications especially military.
The reverse of the envelope carries the CV designation thus demonstrating equivalence between the two designations.
The blue band obscures the internat structure but the anode in this version is a solid sheet folded to shape. The EF91 uses two small plates held together with metal bands.
The thin glass tube envelope is 18 mm in diameter and, excluding the B7G base pins, is 47 mm tall.
References: Datasheet & 1040. Type M8083 was first introduced in 1953.