Literally the third sentence of the original post:
"I submitted the VIN as it appears on the frame and on the corrected title to Marti and received an invalid VIN"
Translated: Marti said the Frame VIN was invalid.
Interpreted: The 11 digit frame VIN is not found in the Bronco database.
So it is what Marti didn't say that tells you what he said. If you enter an otherwise valid search string, and you find a null response in the database query...the answer is "invalid VIN."
It does not mean that the sequence number (last 5) are invalid. It just means that there is not a match to the 11 digit search string. Since the whole thing has to match to be "valid."
Gotta be careful when you say: "Down the line." It's a misnomer. The unique series number (last 5 digits) are PLANT specific, and assigned when the order is scheduled. So yes, there was a dedicated Bronco Line, but the sequence numbers were assigned to the Plant.
About 3 years ago I found a 68' u13 6cyl, the frame vin and glove box door vin were identical except the glove box door read u13 and the frame read u14. I sent both to marti, and the u14 frame vin came back invalid, the u13 vin was legit