The 6CM7 is a dual triode with dis-similiar triodes designed for use a a television frame oscillator and amplifier. The electrically identical 8CM7 has an 8.4 Volt heater and takes 0.45 Amps, both are designed for series heater chain use.
Visually the two triodes look the same.
The RCA logo.
The thin glass tube envelope is 21 mm in diameter, and excluding the B9A base pins is 57 mm tall.
Reference: Datasheet. Type 6CM7 was first introduced in 1955.