While sifting through a tool bin at an antique store near me, I came across a rust try square that looked like it could clean up nicely, so I took the $3 risk.
I sanded the rusted cast iron base with 220, and used electrolysis to remove the rust on the rule. The ruler has a bit of wear on the tips but the edges are dead square. The spirit level in the base is still in great condition. After sanding and scraping the gunk, I gave the base a new coat of gloss black paint.
The blade is a Mohawk with etched markings up to 1/64, the base has no markings so its hard to tell what make it is.