In 1961 Mullard introduced an improved version of the PY32 TV HT rectifier diode. The new valve was the PY33 and it had an improved cathode oxide coating which offered a reduced forward voltage drop.
From the valve museum:
http://www.r-type.org/exhib/aaa0167.htm
The question is, did later production Mullard GZ34 rectifier valves also benefit from the improved cathode oxide coating?
DFWB.