Thanks to everyone who took the time to answer my questions about
the history of Reference Designators.

For some who are on one list and not the other, here is a summary:

Crystal Rectifier was the clear winner for CR, although Current Regulator
works too.

The origin of U was "unrepairable"
(although I don't know how you could repair many other types of components).

Even if the origin of P was "Plug" most people that responded said
(if applicable) J is for the fixed side of a connector, and P is for the
free side,
regardless of gender (interesting)

The most common designation for a sub-assembly was A
(but that traditionally meant "telephone" on schematics, for you old-timer
trivia buffs)

No one knew K for sure, but my pet theory is that when relays are drawn on
the
schematic, the contact touching the plate (normally closed position) looks
like the letter k.
Oh well, enough for philosophizineering, eh?

Well, it was fun, but I better get back to work,
thanks again
Jack