I think all K1100s are the same.
Nope. K1100LTs built before 3/93 have the early seat brackets - same as used on 86+ K75s and K100s. (All Corbin seats use these too.)
Later K1100LTs and all K1100RSs had the later brackets with the rubber insert.
What brackets are you looking for? 3 & 4 in the diagram above?