There's a threaded stud over on the left, next to the Spica vapor canister. It points forward from there, with the front by the left shock, with a wing nut holding it on.
Do you know if this matches the ALFA parts manual for the year of your car.I have a different set up on my 1974 gtv 2000 UK car.There is a bracket coming off the left hand wheel arch (near to the fuel pipe). A threaded bolt is fixed to this bracket. This bolt then goes through a hole on the flat triangular piece of metal on the jack and is clamped in place by a wing nut. I think this is all original.