It's a bit difficult to say why P-valves and E-valves were mixed.
They are very similar but still there are differences.
The PCC85 does not demand as high as anode voltage as f.e. ECC85 which
may make the P-valve the most attractive for this set.
The heater voltage is also a little different but since the dropper resistor
compensates for this, that cannot be the reason.
For the output valve, the anode current and therefore the choice of output transformer
could have called for the PL84 rather than EL84. Worth noting that the P/E L84 valves are rated at
a whopping 12 Watts max output power, where B&O conservatively rates this set at 4,5 Watts.
That output valve should be good for many years in this job.
Thinking of the FM front-end E/P CC85 valve, I've replaced far more E-valves than P-valves
in my time but probably far from enough to produce any valid statistical outcome.
It's not as unusual to see as you may think and certainly, it's very common for a
lot of B&O models around this time to mix these valves.
This JET cabinet is now only waiting for the chassis.
Martin