The only difference int he part number is the last number(s) after the "A" in the part number. I have it @ home but not here but it maybe "A11" and and the regular one is "A9" and you would notice the difference.

The part# that I got was 861523A9 which is the original smaller one. Now if I can get this thing lined up good, I should be ready to go. Wouldn't you know it, it's been 90-100 all week and instead of being on the water, I'm sweeting my a$$ off working on this thing, and my friends boat. We thought he just had a bad u-joint, but it is the upper gears. We pull it apart and 5 teeth are gone, 3 the bottom gear and 2 on the pinion. Time for a rebuild.