From what I can find, the 10Si and 12Si are internally identical. As Kurt says, it has been modified, but I don't see why. The regulator has surely been replaced, since I don't believe GM ever produced any 6V alternators.
You may be able to convert back to negative ground by only making some connection changes. This site may have some useful information (bottom of page) -
https://alternatorparts.com/10si-alt...-and-kits.html