The 12A8GT is a heptode mixer known in America as a pentagrid converter. The local oscillator is formed between grids one and two. The screening grids are g3 and g5 and the signal is injected via the top-cap to g4.
The 12A8 as G and GT versions is the 12.6 Volt version of the 6A7 (UX7) or 6A8 (IO) and other characteristics are identical.
This exhibit was sent to the UK under the Lease Lend scheme during WWII and the envelope says that distribution is by Tungsram.
The wire grids.
The wide glass tube envelope is 29 mm in diameter, and excluding the IO base pins is 68 mm tall.
References: Datasheet, 4059 & 1040. Type 12A8GT was first introduced in 1939.