I found this old key in my collection of weird stuff.
The D.K. Miller Lock Company of Frankford, Philadelphia was incorporated in 1871, and reincorporated in 1904. In 1907, they built a new factory in that city. They were the first to make sliding bar locks. By 1912 they were producing 20,000 padlocks per day! They were sold in the 1930's to the Yale & Towne Manufacturing Company. (For more information, I see that there is a 1986 book on the company's history: Charles Cameron, Illustrated History of the D.K. Miller Lock Co. Padlocks.)