The 6A8G is a heptode or pentagrid mixer and is typical of the 1930/40s frequency changer. The triode oscillator is formed between the cathode and g2 acting as an anode, and has to generate a peak to peak voltage of 25 Volts. The mixer is screened by G3 and G5 with the signal injected into the oscillating electron stream at the top cap g4.
The etched identification is contained with an octagon. The reverse has been overprinted with the Mullard name, BVA and the word Amerty. The later was a name used by Mullard to indicate a valve made for them in the USA. In this case probably RCA.
The top cap connects to g4 for signal input. On the left it can be seen that the screening grids g3 and g5 are strapped together. The central cathode tube is round and g2 (oscillator anode) can be seen to consist of two plain rods where the other grids are notched.
In this enhanced image the etched Type designation is visible. Below the lower mica the two rods of g2 are joined together.
The classic envelope is 38 mm in diameter and, excluding the IO base pins, is 98 mm tall.
References: Data-sheet, 1040 & 1043. Type 6A8G was first introduced in 1937.