Stig Comstedt raises an interesting point regarding the 2D2A.
He comments after looking at our indirectly heated double diode 2D4A that in addition to this 4 Volt valve, a battery version was also made with a 2 Volt heater. Indirectly heated battery valves are unusual.
The 2D2A is a Mullard B5 socketed version of the Philps KB2 with a 95 mA 2 V heater. Mullard had both 2 Volt battery double diodes and mains powered 4 volt versions to offer set makers flexibility. The 2 Volt K-tubes date from the mid 1930s and were issued by Philips with Ct8 and Ct5 bases for diodes and B7 or B5 bases for other types. The K-tubes were issued with and without top caps.
The data sheet for the KB2 shows the same shape as the 2D4A.
Finally the 2D4B has the same diode characteristics as the 2B4A but only draws 0.35 Amp heater current, the 2B4A draws 0.65 Amp.