The 8A8 is from the era of VHF Band III television in the mid 1950's. The valve is a triode pentode, with separate cathodes, designed to function as a mixer and oscillator in the tuner or converter if used with an older Band I television. In the UK Band III television started in 1955.
This Cossor valve has green lettering and the Cossor name as a diagonal flash. Our other Cossor 8A8, also made in the UK has less elaborate lettering.
Looking at the pentode. The grid support rods can be seen through the end wall of the anode.
The control grids of both sections are wound on copper supports.
The thin glass tube envelope is 20 mm in diameter and, excluding the B9A base pins, is 49 mm tall.
References: Data-sheet & 1040. Type 8A8 was first introduced in 1954.