Found this key plate in a parking lot, of all places. Also found a bunch of coins dating mostly 60's and 70's. Not sure if the key plate is antique, or just made to look that way. Either way, it'll look good in my relic stash.
Quick update: turns out it's cast bronze, not brass, and it was manufactured by the PENN Hardware company. PENN manufactured doorknobs, locks and hinges from the 1880's to the 1940's.