and do not leave the output shaft behind!
the splines are shaped differently and must be used as a set, else ZING, you will be walking.
The spline-count is the same so you can swap them around no problem. But the regular 904 splines are sharp compared to the A999, and my mismatch went about 12 miles, when I lost propulsion. Then the hunt was on for what I could possibly have done wrong.