The Peugeot 5-speed was 21-spline, the AX-15 was 23.
For the AW4 auto, the earlier ones (until mid-1989 or mid-1990, I don't quite recall,) were 21-spline, then they went to 23-spline.
You're late enough that I just wouldn't worry about it - get the later 23-spline input. You may be just "short" enough that you can just swap the input gear on your unit into whatever replacement you get, so you can (probably) get a 21-spline as well. I don't recall when that break happened.
What's wrong with the case? May be easier (and cheaper!) to just rebuild it yourself...