Weeds,
There is a neat page that shows ID clues for these alternators...someone will probably post it. The most basic points for ID'ing this are the orientation and location of the terminals for the DA plug.
If the terminals are on the back of the case, noted as R and F and are positioned this way II, it is a older unit that requires an external voltage regulator, which would be mounted on the firewall.
My guess is that it's at least a 10SI unit. This would have the DA plug located around the circumference of the case and the tabs, marked 1 & 2 would appear this way --.
Both styles will have a large BATT terminal. It isn't quite as confusing as you make it out to be. It's either a externally regulated type...older technology and more scarce than the SI, or internally regulated type.
The SI style can use three wires...or one wire. This would be obvious because the hole where the DA plug goes would have no terminals protruding.
My bet is that there is a Delco number stamped on the case that would give all the hairy details on your swap meet find.
Take a look at this link.
http://www.madelectrical.com/electricaltech/delcoremy.shtmlAnd maybe this link.
http://www.rowand.net/Shop/Tech/GmAlternatorIdentification.htmStuart
