HP - when was it used and why?

Interesting.
The information you guys provided probably explains why a 67 A body with a very late build date has HP stamped on it.

So the letter I received from Chrysler with 383-4 HP 8-Cylinder is not the same as the HP stamping on the blocks, but it does mean High Performance.

I didn't think there would be a magic answer that solves everything, I thought it would be, well, what it is.

Thank you FABO.