They look identical to me, main fixings are the same an all. However, my 1100 one just bolted to the rack and ultimately caused it to fracture in about four places, so I replaced the rack with the later beefed up version (£35 IIRC).
The 1150 one I have just fitted was the same plate but has two additional angle irons which run from the rear pannier rail mounts to the back of the rack, so as to support the rack. Bit industrial but should do the job.
Short version - they should interchange no probs, but if you haven't got the upgraded plate probably worth shelling out the £39 for it (though I think there may be one up for grabs just now on the B*W Club site)
HTH
LC