The part which you possibly did not know existed! This clamps onto the hook which is welded to the floor of the boot (trunk). The hook on the floor is available separately as AlfaStop item # 1MW167.
 
Now made in England exclusively for AlfaStop.
 
Another example of the attention paid by Ing. Satta and his team when designing these fantastic cars, there is even a recess for your thumb when operating the clamp... Other makes would have just used a rubber hook, a screwed clamp, or gravity.The height of the hook on the boot floor seems to vary from car to car, possibly as a result of the hook or floor panel having been replaced over the years, or both!Rubber pads applied to the clamp base which contacts the wheel can make up any difference in height, & will additionally provide some protection to the paint on the wheel which contacts the clamp base. The installation photo shows an original clamp in situ.
 
Part Name: Sprint, SS & SZ Spare Wheel Retainer Clamp
AlfaStop Product Code: 1CWF114
Original Alfa Romeo Part No: 1493.46.703
Model: Guilia 101 Series, Giulietta 750 & 101
Quantity Per Car: 1