73 Duster Electrical Issue

Yes the relay pictured is the starter relay. The bottom terminals are the relay coil (electromagnet). The YELLOW wire is "start" voltage coming from the igntion switch. Make sure you have that. Put a light or meter on the yellow wire--just stick it in the connector as it's still connected, and see if you have power when someone turns the key to start

Does it crank? No? No remove the OTHER push-on "dark colored_ wire. This goes to your neutral switch and provides a GROUND for the relay when the transmission is in park or neutral. Take an alligator clip wire and ground that terminal. Not the wire, the terminal on the relay. Try to crank it again. No crank? The yellow is hot? Then you need a starter relay.

On your relay, the "big stud" with nut is the battery power and that is a junction point AS WELL AS one of the relay contacts. The other remaining wire goes to the starter solenoid. So jumpering those two big terminals with anything metal--pliers, screwdriver, etc, should cause the engine to crank.