The 2D4A valve houses a pair of signal diodes with a common cathode. The valve design is from 1934 and the diodes would have been used for demodulation at, probably at around 455 - 500 kHz, the IF frequency of broadcast radio receivers.
The 2D4B has the same electrical characteristics but is on a B7 base and has the greater versatility of separate cathodes.
The reverse has had some paint removed and the underlying etched lettering is made visible. These maker's marks were never intended to be seen and on some valves where the metal paint has fallen off completely the etched lettering is the only means of identification.
The classic envelope is 28 mm in diameter and excluding, the B5 base pins, is 72 mm tall.
References: Data-sheet & 1043. Type 2D4A was first introduced in 1934. See also 1934 adverts.