There was an interesting article in a "Radio Craft" about 1936 or 1937 when the metal tubes first appeared.The particular aspect the article concerned itself with was the temperature of the glass versus metal tubes - which ran hotter,which dissipated the heat more effectively/efficiently but also other aspects such as inter -electrode capacitance differences if I remember correctly.I got the feeling the article was written by someone independent so it was not an advertorial.
As with other comparisons between old and new versions of things I would expect the newer item to have benefited from advances in technology. So this does not surprise me the fact that a tube made in 1936 might not perform up to the level as a tube made in say 1965.
But I will say that I had never noticed the differences you mentioned spelled out in a manual.
Thanks,
Henry
Russ
Both tubes have max Plate Voltage for amps as 300V.
The difference of 100V and 250V is only for Class A1 amp, that is where the grid is negative. Not for others.