The 12Y4 is a pair of signal diodes in a classic envelope fitted with a B5 base. It dates to the early 1940's.
Oliver Palix comments at radiomuseum.org: as with the ARP3, the 12Y4 is used in the WWII Canadian set C52. One for AGC and signal detection, the second as a noise limiter.
The Marconi C52 short wave transceiver was designed for operation from a 12 Volt vehicle battery with a rotary transformer to generate the HT supply. Coverage was 1.75-16 MHz and this medium range set had an output power of 70-100 Watts CW or 45-75 Watts voice.
The signal diodes side view.
The pair of indirectly heated diodes face view.
The classic envelope is 38 mm in diameter, and excluding the B5 base pins is 88 mm tall.
Reference: 3002. Type 12Y4 was first introduced in 1944.