The DH719 is a triple diode triode. The triode and two diodes share a common cathode but one diode has a separate cathode.
The three separate electrode systems can be clearly seen, and within the right hand anode can be seen the cathode and control grid.
The Marconi identification is by a paper label for extra impact.
The DH719 was specifically designed for use in the detector stage of early FM and FM/AM radios. The triode section providing the first audio amplification, the single diode and one of the diode pair were used as an FM detector in a ratio detector circuit. SeeFM Broadcasting and Signal Detection. The other diode in the pair being used as the AM detector when that part of the circuit was required.
The thin glass tube envelope is 20 mm in diameter and, excluding the B9A base pins, is 58 mm tall.
References: Data-sheet & 1040. Type DH719 was first introduced in 1955. See also1955 adverts.