There are two pieces of sheet bent to form a "U" shape bracket in the middle to hold the pivot pin. These are spot welded through the boot floor to a stiffener plate underneath. A split pin passes through one of the brackets (they both have the hole) to secure the pivot pin. The pivot pin secures an eye bolt that the clamp fits onto, which is in turn secured by a flat washer and a wing nut. The pivot pin is originally 1/4" dia x 2" long with a 0.094" dia cross drilled hole 13/32" from one end (and using a 0.090" diameter split pin).
 
Photo at right supplied by Mark Hester in Perth, Australia (or London, UK). This pivot pin recently supplied by Moss Motors UK is about 1" or 1-1/16" long with no cross drilled hole. Moss are saying they have supplied the short pin since 1991. Larry Wheeler in Oregon, USA bought Moss Motors USA part number 453-025, and it is the same incorrect part. This is definitely wrong. The only way to use it is to sandwich it between two split pins that will bear against the ends of the hinge pin. This leaves the hinge pin supported (or held down in this case) by less than 1/2 the width of each floor bracket. The spot welded joint between these loop straps and the boot floor is a common rust failure point. Strength of the joint is significantly compromised by the short hinge pin causing a twisting load on the floor brackets.

On 25 Jul 2013, Mark Hester wrote:
"I drove my car over a speed hump and the spare wheel made a loud bang, so thought it best to try and fit your part anyway. .... If you wedge this pin in with two split pins, one either side, if there is no load on the spare wheel retaining spike the pin simply falls below the split pins and falls out".
Here is a picture of the original pivot pin for comparison.

On 18 Sep 2018, Eric Von Wenckstern wrote:
"I found a 1/4 in. x 2 in. Zinc-Plated Universal Clevis Pin from Home Depot worked as a great replacement. And it only cost a $1.73! It’s not for the concurs buffs, but for the average Joe it’s cheap and effective".