Hi Joso,
I think I may be interested in your "spare mounting bracket", but am not sure if this is the plate or mounting bracket (don't know the correct names) that bolts into the holes where the grab handles bolt into the rear panel of the bike, onto which I presume the top box base plate mounts. Or is the mounting bracket also the top box's base plate? Apologies, but I haven't see an original ST1100 top box mounting arrangement so it is all unknown at this stage...I will have to see the parts and see how they can be adapted to the Valdera Top box and base.
This is my first post so apologise if this is not how it is done.
Cheers,
John