Hi,
If it is the rivited hook I think you mean, it is a tongue of metal about 1" wide by 3" long with the end section folded back on itself to form 3 sides of a square about 1/4" on a side.
If you take the black section of the pannier with the rail mountings away from you, there should be a black tongue on the front edge pointing towards the front of the bike and slightly down. The bracket rivets onto there with the curled back hook towards you.
This slides into a slot in the mounting rail and the locking latch on the rail secures it on that hook.
I would be real careful about using the panniers. There is a fair amount of additional steelwork that went into the back of the ST as part of the pannier kit. It is there to strengthen the frame back there to take the load. Somewhere on this site is a good set of instructions with diagrams for the extra steel. It might help you identify if you have the steel onboard, in which case all you are probably missing are the rails and maybe the stalks to move the rear indicators.
You can find the instructions here.....
http://www.triumphnet.com/ Look under 99-04ST>FAQ>Inst/mods/Upgrades>2 box Pannier
Rexx