There are two types of starter relays that I've seen. One has the battery post and a screw-down connector for the starter wire and two spade terminals, one for the wire from the ignition switch (marked I) and one for a ground wire (marked G). The other relay has an extra spade connection that is all by itself, and I have no idea what it's for. I've always used the four-connection relay for my cars.
Typically, manual transmission cars prior to clutch pedal safety switches had one of the primary terminals grounded to the starter relay case, as pictured above.