The 12A5 has a conventional indirectly-heated cathode but its heater is centre-tapped so that it could be used in (American) car radios suitable for either 6V or 12V systems. Car radios of this era used 'vibrator' HT power packs which were not very suitable for generating HT voltages in the 250-300V region. The 12A5 was therefore designed to operate well from lower HT voltages and therefore needed a 'juicier' cathode to provide extra current.
The Impex logo and the word Foreign.
The dome and top mica components. Note that within the three wire grids are two cathode tubes. Each cathode is threaded with two hairpin heaters making four pass in total per cathode.
The UX7 base pins.
The classic envelope is 38 mm in diameter and, excluding the UX7 base pins, is 91 mm tall.
References: Data-sheet & private communication. Type 12A5 was first introduced in 1940.