Thanks for the eggcellent pics! Yup! That is the rare t/case input only used with a L43.
Grind something along the lines like this. You don't need to be precise as the transmission output coupler never even comes in contact with much of these splines past this stepping. So grind away while rotating the input (trying your best to keep the front bearings and holes covered and blast the metal away from the t/case), and double-check the fit frequently as ideally you only need to grind away the minimum amount needed.
BigMike
