[TimG]

Here's what I've heard about broach marks. As the broach machine lost its sharpness, the cutting edge or edges were sharpened. When the broach was sharpened, they would surface the hi performance blocks first, this could mean that they would surface the four bolt main blocks first since they were going to have a higher compression.
I don't know if this is true or not, but the broach certainly would wear down leaving different types of broach marks. I've always noticed more of a pattern on big blocks than small blocks.