I picked up this square at a yard sale recently. It cleaned up nicely, first in the sandblast cabinet to get rid of the heavy rust, and then on the wire wheel, and finally wet sanded.
"Aetna" was a trademark of the Southington Hardware Company. In 1930, the Aetna No. 200 sold for $10 per dozen:
(Catalogue page and photos above from Toolemera.)