This arrangement is unique to the MGA 1600 Coupe model, having a solid rear bulkhead with the spare tire contained entirely in the boot. This makes for more luggage space behind the seats at the expense of a little less space in the boot.

Notice the bulkhead fittings with the front hold down clamp and "U" shape brackets to hold tool roll straps, and the tire clamp relocated to center at back.

Photos courtesy of Gordon Pugh
Find more information on the hold-down clamp in article BD-135

Addendum 7/9/2014:
The following pictures are supplied by gary Starr in Illinois, USA.