The main or apparent difference is the earlier AD trucks have a large fuse box on the firewall with 5 fuses.
The later AD trucks have a small box with only 2 fuses.
I believe the earlier harness include fuses for high beam, low beam, and park lights. The headlight switch also has a circuit breaker that (I believe) protects this wiring.
The later harness without these fuses still has circuit breaker in light switch.
Unless you're real concerned with exact originality I don't see as it makes much difference either way which version of the harness you use. So long as you have the appropriate fuse box to go with it.
