I've wondered that myself - especially the 3 3 ones. I suspect that they made them in btaches. I actually have a 1956 (without case vent) & up case with a 3 3 casting date. That tranny case was almost 3 years old before they got to machining it.
THANKS . HAVE THE SAME CASE . I HAVE A ORIGINAL 1954 WITH A 3 3 CASE. I THINK IN THE EARLY FIFTY'S HARLEY MUST OF HAD THOUSANDS CAST.JUST AQUIRED A 1956 WITH A 3 3 CASE