It is the positive that does the work Bushy, not the negative except on real old cars, mostly from England although early Holden's were positive ground as well. If you put an earth on the threaded terminal and have the solenoid mounted to the chassis you would have to be running a positive earth system and the electric starter motor will turn backwards and not engage.
Yours could be some weird thing that someone has rewired to be positive earth but it is pretty unlikely. Take a pair of jumper leads, hook one to the negative and the other end to the chassis. Hook the other to positive and touch the other end of it to the terminal on the starter motor. If it engages the gear drive and turns over the motor, it is a negative ground system. If the starter just spins but the gear stays down then it is possibly a positive earth but again, highly unlikely.
