The 2A6 double diode triode is designed for detector and first audio stage in radio receivers made for the American domestic market. The heater voltage of 2.5V was the original (late 1920s) standard for American indirectly-heated AC mains valves.
Later variants were the 75 with a 6.3 Volt heater and the 6B6G with 6.3 volt heater and IO base.
The valve has a common cathode assembly, seen running vertically through the centre of the valve. The signal diode anodes are the bright plates seen in the centre of the envelope. Each side is insulated and forms a diode anode. The top of the electrode structure is the triode. The anode cavity is blackened.
The valve carries the Belgium ADZAM brand and also a paper band around the base. Like other European companies in the late 1940s with degraded manufacturing capability, Adzam were re-branding American made valves.
The classic envelope is 38 mm in diameter and, excluding the UX6 base pins, is 100 mm tall.
References: Data-sheet, 1040, 1043 & 3002. Type 2A6 was first introduced in 1933.