I just bought a very nice 1975 400F on bringatrailer, it came with one unnumbered aftermarket key, which works fine in both locks. I found the lock numbers - they have different numbers, but the last three numbers are the same. (For security I use 123 here, the actual numbers are not that.)
The number on the ignition switch is 7GA123
The number on the seat lock is 7FI123
Is the key number one of these numbers, or something related but different?