Chryco, the S/N is on the block by the oil pan and matches all the other places. Although they match the other numbers in size, I believe it could have been someone who re built it, that may have added it later once they made it an HP. Doesn't explain why the orange paint unless they boiled the block and got rid of all traces of the turquoise paint. That makes more sense to me than anything else.
Barry, you mentioned this a while back and I spaced it, no problem, go right ahead.
Here's a picture of the ID pad. You can hardly make out the HP by the date.