Interesting pics Mark...I think that's the same 69 bench seat photo I remembered seeing posted somewhere here before. I'd be very curious to see what the interior trim code would be on the body tag. I'm still going to have to assume that's a 67-8 seat with a 69 deluxe style trim shop upholstery job. It would be easy to tell if it were original upholstery if you could look at the stitching, piping, vinyl material, and grain pattern in person, but in a picture one can only speculate. Interesting though for sure.
As for the "Cohen" Motion Camaro, I'm betting Marty was remembering the stark naked trans tunnell in a no console/column shift car, and simply confused it for a bench seat. That was a pretty rare/cheapie looking combo in a 69 Camaro, especially considering the highly optioned cars Marty would be used to seeing roll in and out of the Motion shop. I think he simply remembered "plain and cheap" and equated that to a bench seat, not even realizing that it was an extra cost option in a Camaro and a rare/non-existant one in a 69 car at that.
Also, that looks like a Pontiac tach to me...definitely not the shape of a Dixco housing. Motion used a lot of Pontiac tachs on their cars and simply covered the "Pontiac" lettering with a "Chevrolet" decal on the lens. There's close up pics of one in print on a DY Motion prepped 69 RS Z/28 that I'm sure Joe G can find for us.
GM also marketed that Pontiac style hood tach as an over the counter item with an AC logo. Perhaps Motion used those as well, but I don't remember ever seeing any on Motion cars in print.