I agree, all I can do is draw conclusions from what I have seen and keep seeing. Of the 40 or so jacks that I have right now about 4 dont have date codes and those in my opinion are early 67 jacks before they started doing date codes. Its amazing when you start looking at them really close you can eventually find date codes on most of 1967+ jacks. But I will not dispute that some of them may have been missed or not stamped.
In the end if I was doing a restoration I would want one that is date coded for my car and the correct style....thats what it really comes down to.
If all goes well Mustang Monthly will be running some of this information in May in their magazine.