Tony,
I'm not sure where you got your information from, but I don't think it's correct. I've never heard of this and there's nothing in any of my reference material mentioning the "HI" or the "HIP" codes. "Chevy By the Numbers" has no info on these as well.
The "P" was used on the Muncie 4 speed (aluminum case) starting in the late 1963 model year (for Corvettes), up through the 1974 model year.
Although there are a lot of good WEB sites with Muncie info in them,
HERE'S ONE I found doing a quick Google search. Others might chime in with more.
You should also pick up a copy of Alan Colvin's books, "Chevy By the Numbers" series. They have a TON of great info in them, and Alan's even a member of the
Camaro Research Group.
Ed