My only experience with the those props was on an Evinrude Sweet 16 with a 65 90hp. The 10 X 9 1/4 gave the same top speed and rpm as the 9 1/2 X10 but it had a better holeshot. Even with the 23:20 gearing the 200rpm/in pitch change yardstick still held, when I used the 10 X 11 the rpms dropped 400. I have seen quite a few 10 X 9 1/4 on ebay, but no 9 1/2 X 10s. iboats prop selector is of no use for these old V4s, seems to think they used props ranging 13 - 25 in pitch