................ How would these cars be treated by MCA judges as far as originality of seat material? I know of three other late production Mach 1's that have the same all vinyl (no comfortweave) material.
IF they didn't match what their (the judges) experiences have been you would likely get a deduction or a comment on the sheets with no deduction if they were unsure. How We've handled "new discoveries" in the past when I was an Assistant Head Judge in charge of a class is that the new info was introduced at the judges meeting, on forums where the judges for those cars participate or at least lurk
and ask that the owner offer the documentation and information during the introduction of the judges at the show.
At MCA and SAAC teams have been asked to, during their introduction and before starting judging, to ask the owner/representative if there "is any thing unusual or different about their car that would like to mention"
This was designed for this reason and should allow you to respectfully offer your findings before the judges get about their task of judging.
Just my suggestion based on what has worked for me and many owners I've talked to over the years.