Im not sure of the answer but I have had a couple of dist for my Z and they all have had a rectangular indentation around the part number and not really the date. I had a Jan 70 i have a may 70 (which I dont have a photo of right now), and just tried to by a march 70 (dang thats the one I need) all looked the same, they all had the same lathe type markings. I have noticed that the stampings do not all sit in the same location vertical to the housing,. Here is what they looked like (sorry none with 4s in them).
I wonder why some have the pronounced groves and some dont.