The 12H6 envelope contains two separate signal diodes.
The 6H6 is electrically identical, they differ in heater Volts only. These double diodes were provided for a range of applications from signal and AGC detection to low power rectification, clamp diodes etc.
An American television textbook of 1950 uses these valves extensively as level clamping diodes.
The can is also marked with the US service code of VT-214.
This is an RCA made valve.
The thin metal tube envelope is 25 mm in diameter and, excluding the IO base pins, is 29 mm tall.
References: Data-sheet & 1040. Type 12H6 was first introduced in 1940.