[Woodz428]

I was wondering how they came up with 10, I was thinking of engine families. Including the flathead, I'm not sure there are 10 "families". The flathead probably wasn't included because it was focused on HP production engines. There were no factory HP flatheads, although the aftermarket tweaking of the flathead laid the groundwork for production HP engines.
I counted how many engine families there are(Ford,Lincoln and Mercury, excluding truck engines) and I think that excluding the 6s and 4s of various configurations, there are 10.