Photo below (compliments of John Terschak in Kill Devil Hills, NC, USA) shows an original installation of the MGA bonnet latch release return assist spring. I'm pretty sure this picture does show the original parts for the spring and tab on the rod. And yes it does simply hook onto the threaded end of a fender bolt. I don't recall ever seeing an original installation up close, not even "back in the day" when I had three basically original roadsters in the late 1960's. (Likely I just wasn't looking at these particular parts back then). My trained engineering mind thinks the tab and spring should be installed one or two bolts farther forward where it would be more horizontal and pulling more in line with the rod for a bit more forward force and less side load friction (but maybe just an opinion). $0.02

Having driven my present MGA more than 235,000 miles without this spring, I can tell you there is no way this rod will ever rattle. The two slotted guide tabs on the inner fender force a long bow in the otherwise straight rod. This preloads the rod like a long leaf spring against all of the mating points at the bulkhead plate, two guide slots, and the connection at the front bell crank, so it cannot rattle.

The assembly works okay without this return assist spring, except it does NOT return by itself due to friction in the three sliding points. I have developed an ingrained habit of pull/push to release the bonnet latch and return the mechanism to rest in one second. If you pull and let go, leaving the mechanism in the retracted position, the bonnet will not latch when closed until you go back and push the rod forward again. That's easy enough to do in the engine bay while holding the bonnet up with one hand.

For may years (decades even) I have been meaning to install this spring when the opportunity may present itself. Apparently the non-retract problem doesn't bother me much, as it has never been high enough priority to actually do it.