Hi,
The "PR" version of a tube is mechanically identical to the non "PR" version except that it has gone through additional electrical testing to ensure the high voltage stand off required for Pulse Radar service among a few other tests. You will notice a different set of specifications on the PR tubes data sheets since they were tested for specifications required for radar service. Tubes, like many solid state devices today, had their final specs determined after manufacture. For our service, you will not see a difference in a PR tube from a non PR tube.
It has been said that the pulse rated versions of ceramic tubes have different spacings between electrodes... 3cx800 vs. 3cpx800.
Dunno, don't have the calipers anymore, and probably couldnt see them if I did.
--Shane