[Kube]

Robert, I certainly respect your thirst for knowledge.
In this case, I believe any information you gather will prove ambiguous at best.
There are many variables that would make pinpointing when the early latch was replaced by the second "late" latch.
If you could get your hands on the engineering drawings and release dates, that would most likely narrow down the date "window" about as good as I'd expect possible.
When Ford made a release for production, some (all?) assembly plants used up existing stock. Other plants (unless specified otherwise) may have returned their inventory. And there may very well have been other plants that even though directed to return stock, used the parts up anyway.
Also, one must think about which plants were installing which engine / transmission assemblies. So, no exact correlation between latches and serial numbers...

I have witnessed a number of parts, specifically 1940 that were released for production for Job #1. In other words, the very first 1940 to be built. Yet, there are examples of these "Job #1" parts not entering regular production until thousands of cars had already been assembled with earlier designed parts that were designated "not for production".
Was that one assembly plant? Numerous plants? We will never know.