Not certain what you are referring to as a "PK45" ???
The second letter in the model number refers to the the trim level in the line.
To the best of my knowledge there's no "K" designation in any mopar model line.
The ones most commonly used by mopar throughout the years:
"L" Low
"M" Medium
"H" High
"S" Special
Now specific to the 1971 Plymouth Fury line the following models where available:
PE - Fury I and Custom
PL - Fury II
PM - Fury III
PH - Sport Fury
PP - Sport Fury GT
And lastly, this post should probably be over in the wagon section not the forward look
I was aware of "N" and "T" model lines for taxis but didn't include the info in reply since they are not as common.
For whatever reason I've never noticed the "K" designation on actual ordered police cars. Maybe too focused on high performance models but, now learned something else.