Thank you torontoworker-that helps a lot.
I have attached the invoice on the engine replacement-they call the new engine an "exchange engine" whatever that means. That makes me think that this was a factory warranty job.
The replacement engine's number is in the usual place on the bottom of the case-but it is very lightly inscribed with a hand held tool, not stamped like other engines that I have seen.
Can you point me toward any published articles or reading on this subject?
Thanks again!