OK, just following up on my original post. Looks like the engine is the original and not a replacement (as far as I can tell) given that it has a M96/03664xxxxx engine code, and the 997 water pump can actually be used on it according to Loren over on a Renntech post (
http://www.renntech.org/forums/topic/41751-m96-and-m97-confusion/page-2). Either the factory installed a 997 pump on the engine originally or it was replaced with one at some point (I'll ask my mechanic tomorrow if he had to snip the gasket).
In short, feel free to ignore my original question
.