65amp wiring/trouble shooting
The missing or contradictory information is the other end of wire C13 12 BK (power feed for relay).
According to the main diagram it should be shown in the drawing for CI 5 (ammeter terminals).
On Mark's car, which is equiped with the 65 amp option, which is probably wired the same as 60 amp, we know the grommet carried a second black wire around 12 ga., which joins a second brown wire that is likely welded to the alternator output.
I think the only definative answer will be on the cars themselves, or if available, assembly drawings and service bulletins for 1974.