[julie_m]

I hope this might help someone:

Handy Guide to Mullard Type Numbers (almost certainly incomplete)

First letter: Heater voltage or semiconductor material.
A = 4V or Germanium
B = Silicon
C = 200mA
D = 1.4V
E = 6.3V
G = 5V
H = 150mA
K = 2V
O = cold cathode
P = 300mA
U = 100mA

Second and subsequent letters: Electrode structure(s).
A = signal diode
B = two signal diodes sharing a cathode
C = signal triode or AF transistor
D = power triode or AF transistor
F = signal pentode or RF transistor
H = hexode or heptode requiring separate oscillator
K = octode or heptode used as self-oscillating mixer
L = power pentode or RF transistor
M = indicator tube
Q = enneode
T = thyratron or SCR
Y = rectifier diode
Z = two rectifier diodes sharing a cathode, or zener diode

Tens: Package (only for valves).
0 = Continental side contact
2 = Loctal
3 = International Octal
4 = 8-pin Rimlock
7 = wire-ended
8 = 9-pin miniature
9 = 7-pin miniature

Units:
Usually 0 for the first valve of a type, then increased as variants were introduced. Often, an odd number on a pentode indicates variable µ.

So for example we have DM71 (a wire-ended indicator tube with a 1.4V heater), UCC85 (a 9-pin dual triode for 100mA series heater wiring), GZ34 (an International Octal full-wave rectifier with a 5V heater), EABC80 (a 9-pin multi-way valve with a triode, a separate diode and two more diodes sharing the triode cathode for 6.3V parallel heater wiring) and BD139 (a silicon audio power transistor).

The system isn't perfect ("EB91" actually has separate cathodes; some valves had multi-rated heaters, e.g. 6V3 / 300mA, and could be used in a series or parallel arrangement, but this is not obvious; and it totally lost the plot on the numeric portion with semiconductors) but it's at least an attempt to be sort of vaguely logical.