These were made to the exact shape and contours of the GT exhaust ring, doesn't mount flat.
Only motorboat these fit must have a '65-'66 GT Mustang rear valance.
Guess you've never seen another set either.
Nope - have seen others as reported - never said that had a flat base. Marine application would not automatically mean that a flat mount was necessary. Wood boat such as ChrisCrafts come to mind
Was told/suggested they were from a boat application - could be from another source but have seen then used on 65-66 Mustangs
If they were the "exact" shape then they would be allot flatter without the extension as shown. Guess its been a while since you've seen an original GT exhaust ring