Originally Posted by dragonracingteg

The PR4-5 is obviously looking better, but aside from that. On the left hand side of it, it has 4 markings. It might mean something??? Especially since somebody thing the PR4-1's markings should be different! It probably is nothing tho...

Here's what I found anyways...

Basically the PR4 is just saying what type of engine it's for,
the -1...-5 are the series for those types of heads
so far you should be able to just swap them and use what you like.

Deciphering the numbers on a Honda Head...
http://www.honda-tech.com/zerothread?id=721248