Hi Simon
I fitted two catches just like those. They came from the local scrapyard as standard bonnet catches. (Can't remember which car, but most are very similar). On my KVA, there were already two support brackets on the chassis which had been used by the previous owner to fit the vertical pin type which protrude through the rear body when closed and are secured by a spring clip. No way would these pass the SVA, so they were replaced. On the underside of the rear body, I made some aluminium box section supports for the spring loaded prongs. These spread the load to the fibreglass. It's all a tight fit (isn't it funny how with such a big car, everything is a tight fit!).
To release the catches, two small levers pass inside the car and are just slid sideways.
Hope this helps
Dave Tickle