Lots of web site feedback related to preparation of these seat back hinges. The recommendation is to cut off a piece of hinge 20.5 inches long. Then make certain that what you prepare matches what is shown on page 21-07.
As it happened I had some spare hinge (from previous "incidents") so I just made four parts 10 inches long.
Picture 1 shows the hinges before drilling rivet holes, apart from one pilot hole. The bottom hinge is turned over to show the line to be drawn for alignment of subsequent rivet holes. The line doesn't need to be too thin (I used a fine point Sharpie pen) but does need to be centered on the pilot hole and parallel to the hinge edge.
You need to cut out two "eyes" from each hinge. My hacksaw just fit between eyes but need to be angled slightly to avoid interference of the tool frame with the part.
Picture 2 shows the same parts with holes match-drilled to the seat floors. Note that the front hinge on each seat has two more holes than the rear hinge. (I later discovered a mistake - I had done one of the front hinges upside down. They are mirror images, not identical!).
Not shown in pictures is the work to dimple holes in the seat floors.
