39 Mercury Hood latch pictures

[Model51]

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Just like Kube mentioned, the exact date of a change can't be determined for certain for the reasons he mentioned, and I agree completely with his analysis.
If you want to have the official Ford record, then you should request the Engineering Releases for the specific part number of the 1939 latch. If I read the tag from one of your images correctly the part number is 99A-16892 (sorry if I don't have the model number correct, it's out of my knowledge base). You can request the Engineering Release cards for this part number from the Benson Ford Research Library. These cards should describe the official date the parts were released for production and for which models. They should also have dates that changes were made. You can also ask for copies of the Engineering Drawings ($30 each) but the drawings are typically the final version of the part, not the various earlier versions. Realize that for the reasons Kube listed, just having the dates from these cards doesn't mean that the part was used exactly on or from that date, but at least it will give you an indication and some data from which to make some assumptions as assertions as to when a part was actually intended to be used on a vehicle.
Here's a link to the BFRC archives and how ask if Engineering Releases and Engineering Drawings are available for a specific Ford part number:
https://www.thehenryford.org/collect...earch-services