This is a very rare 70's Krauser seat rack initially designed for the R90S / R100S - before 1979 when BMW added their own rack to the grabrail.
It will fit all S seat model that don't have the rack though.
Fits under the existing grabrail fittings & was supposed to be secured at the rear by 2 bolts, but most people used double sided tape instead.
The rack is solid, but the chrome is a bit dull due to 48 years of polishing as it was originally fitted to my 1978 R100RS when new!