The CV138 is a high gain, high impedance, variable mu, screened pentode. It could be used from AF to VHF up to about 200 MHz and was found in large numbers in most types of electronic equipment. In RADAR, radio & TV it was used for RF and IF amplification. It was also used for oscillators and frequency multipliers. The commercial equivalent is the EF91.
The original CV138s were made in the US in the mid 1940's long before B7G valves were made in the UK.
The envelope is silvered to prevent electron bombardment of the glass releasing gas.
This exhibit is dated September 1956 showing that it was in production for at least a decade.
The thin glass tube envelope is 18 mm in diameter, and excluding the B7G base pins is 48 mm tall.
References: Data-sheet & 1040 Type CV138 was first introduced in 1947.