I took a look at my Fastback last night (1970 Mk II). The triangle bracket that drops down from the rear of the battery tray looks like it's held onto the tray with a single 5/16" bolt. I believe the horn was originally attached to the bottom of the triangle bracket. On my bike it is assembled with the bolt inserted aft through the tray, through the bracket, then a lock washer & nut to top it off. But that bolt is long enough to extend past the tightend nut and through the rear fender. It's topped off with a rubber washer, fender washer, and a Nyloc nut. Only one thread extends past the Nyloc so it doesn't dig into the tire. There probably is another rubber & fender washer between the fender & the first nut too. Dollars to a donut this is not the original set-up (rubber washers, fender washers, Nyloc), but rather something I put together during assembly back in '97 & 20k miles ago. "Look good, last long time!"
Hope this helps