According to the parts fiche cross-reference, the hinge bracket fits these models. It also shows that it's still available from Honda for $9.04.
http://www.cyclepartsnation.com/pages/oemparts?aribrand=HOM#/Honda/CB550F_A_(76)_MOTORCYCLE%2c_JPN%2c_VIN%23_CB550F-2000003_TO_CB550F-2010866/SEAT_%2b_TOOLCB500 A MOTORCYCLE, JPN, VIN# CB500-1000001
CB500K1 A MOTORCYCLE, JPN, VIN# CB500-2000001
CB500K2 A MOTORCYCLE, JPN, VIN# CB500-2100001
CB550 A MOTORCYCLE, JPN, VIN# CB550K-1000001
CB550F A MOTORCYCLE, JPN, VIN# CB550F-1109886
CB550F A (76) MOTORCYCLE, JPN, VIN# CB550F-2000003 TO CB550F-2010866
CB550F A (77) MOTORCYCLE, JPN, VIN# CB550F-2100001 TO CB550F-2114871
CB550K A (76) MOTORCYCLE, JPN, VIN# CB550K-1230001
CB550K A (77) MOTORCYCLE, JPN, VIN# CB550K-2000007 TO CB550K-2015308
CB550K A (78) MOTORCYCLE, JPN, VIN# CB550K-2100001 TO CB550K-2122082
CB550K1 A MOTORCYCLE, JPN, VIN# CB550K-1200001