What is this metal door in my basement?

Madison, WI. Home was built in 1923, renovated in the 1960s. We have this strange metal door in the basement with an odd chain mechanism attached to it (second pic) that we can't open or see beyond. The house was renovated to be a multi family home so there is a basement unit, ground floor unit, and second floor unit. Do you know what this metal door is/could be?