Here's an old post with lots of photos and descriptions of the different types of bowls. Even got some JC to split conversion stuff going on there.
I think to summarize, the B bowl has fins on the outlet side that are recessed a bit, and the G's fins on the outlet side are flush with the end of the bowl. Not sure what the dealio is with the R bowl.
Bowl stuff (http://www.***boat.com/forums/showth...=jc+conversion)