Okay, I did a bit of looking around, and here's what I came up with.
The left and right helmet locks on the KZ1300 don't bolt directly to the frame, but to brackets that are. The one kz13a has is for the right side, while the one Craig has is for the left (obviously). Incidentally, both of these are available as reproduction parts on z1300.de. Look for items MCG 320741009 and MCG 320741020 under "suspension and frame".
As for how they attach? Well, this is my best guess.
See, here's what gets me. If the KZ1300 was supposed to have helmet locks from the outset, you would think that there would be mounting points welded to the frame. Both of these brackets are piggybacked off of already existing mounting points- the rear shock and cowl for the right side, the lifting handle and main switch cover for the left. I'm curious if the helmet lock(s) wasn't added until later model years, and rather than change the way the frames were welded, Kawasaki went with the quick-and-dirty solution of just fabbing up a quick bracket and bolting it to existing points. However, this is nothing but pure speculation on my part.
In any case, I hope this helps.