GregZ wrote: You would need a complete seat latch assembly like this
but it appears someone cut off frame lug on your bike that the latch fits into.
Your screwed
^^^^+1
What Greg says.
be careful when you are adjusting the latch, we had to work diligently on mine to get it to close correctly when we did the resto mod on my bike ( 75 Z1-B also), My bike was missing everything when I got it, several ebay buys later had everything. We ended up taking everything apart, thoroughly cleaning it and reassembling it.But it was a tedious task for us believe it or not.
Good Luck
Ed