Starting w/ '67 'Vettes, the top of the gas tank is the location where the build sheet should have been placed at assy. . Sometimes you can peek around a fuel filler boot and see if it's there before dropping the tanks...not sure how well that would work w/ the Dodge style flip filler on your car? . You may be able to remove the filler top assy and look then...or just do it, drop the tank and what's left of it is either there or it's not? .
On the flip caps themselves, I think I've read at least one Motion car was known built w/ a style Dodge filler, not the Enot's/Aston Martin style...other side of the coin though, the Dodge one's would've been the easiest to obtain back in the day for private built cars too?. .
Good luck w/ whatever turns up...if anything looks truly positive, pay the man and spin the wheel!? .
~ Pete