I reviewed the posts you listed including the post below. But, what do the rounded corners look like? That is what I did not see.
Attached is a Pic from an after market Door Sill. Are they rounded?
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Many of the items people thing of as 64 1/2 details were really cover allot more months than just that period. One for wxample is the ealry style sill plates.
At San Jose they were used into December production.
Just as many running changes or more in the 67 production year but they were dealing with more engine sizes and Thermactor on some. OF 65 they were just working the bugs out on a new model of car - lots of changes was typical.
Here is an example of the early squared corner sill plates [PIC WOULD NOT COPY AND PAST]