The mounting is different. The S/C Rivs (Series I) use the mounting ears on the top and bottom, and leave the one on the side unsused. The N/A Rivs (Series II) use the side and bottom mounting ears, with no ear on top. Otherwise, as near as I can tell, they are identical.
I ran into this when I thought I hit the jackpot and bought a fresh reman off a car in the pik-n-pull, thinking the same thing.