Captain Round of the Marconi designed the Q valve in 1916 and it was the first hard vacuum valve designed in Great Britain. The Q had a high anode impedance and low anode capacitance. The mesh grid made it difficult to mass produce in a factory setting and M-OV redesigned it along the format of the later, and lower performing, V24 and the resulting valve was named the QX.
The QX had about half the mutual conductance and anode AC impedance of the Q but had the advantage of working well at lower anode voltages than the Q.
This Osram valve has the brand name etched into the glass as well as the second BBC stamp that dates this valve to between September 1924 and 1927.
The QX has a tighter grid winding pitch than the V24.
With a little enhancement the Osram name can be more easily seen.
Beside the QX designation is the work RECTIFIER, this relates to signal rectification (demodulation) not to power rectification.
The thin glass tube envelope is 17 mm in diameter and, excluding the base pins, is 72 mm tall. Across the side pips measures 23 mm.
References: 1003 & 1005. Type QX was first introduced in 1919. See also 1919 adverts.