Colt SAA's were never produced by Colt with brass grip frames. You see them in movies a lot, but they did not exist. See RL Wilson's big Colt book for reference.
Colt dropped the production of brass grip backstraps before the start of the Civil War. By the time the SAA came to be, all factory frames were iron.
Probably gonna shoot myself in the foot but would that include the 1872 open top Navy grip? Mine has a brass backstrap