The DT215 is a double diode triode on a B5 base. The Hivac Type DT215 has the B5 base but the Triotron DT215 is given as using the Ct8 base.
The double diode triode is the standard superhet valve for decoding the AM signal, rectifying the AGC voltage and providing the first stage of audio amplification.
The reverse is plain metallising paint with the word Foreign denoting an import to the UK.
Looking into the top of the envelope is not enlightening, just the top mica sheets are visible.
The classic envelope is 45 mm in diameter, and excluding the B5 base pins is 108 mm tall.
References: Data, 1043 & 3002. Type DT215 was first introduced in 1936. See also1936 adverts.
