As you can see the valves
were originally: MVS/PEN, 41STH, MVS/PEN.B, DDT, 4XP and 43IU
so the output valve has been changed from a Cossor 4XP to an
equivalent Mullard PX4, either capable of over 4 watts output.
The output valve is a little odd because it has a B4 base and
a directly heated filament which of course requires to be grounded
to act as a cathode but with that connection being arranged so
that the AC voltage across the filament does not introduce hum
on the audio. It also requires to be negatively biased to determine
the anode current.
Interesting is the spec for
the dial lamp which is 6.3 volts but run from 4 volts presumably
for longer life and more subdued illumination. I'm not too happy
about the external loudspeaker rating. This implies that the
speaker should have a built in transformer which means the wires
will carry at least 250 volts DC. The warning will be to preserve
the life of the output valve as its screen would light up like
a lamp if the external speaker is plugged fully home with a break
in the wiring! Using headphones, naturally of the high impedance
variety would be frowned on nowadays because their connections
might give the user a nasty shock.