Sweet lock! Mint (or near mint)-in-box locks are always great.

The address on the box uses the old postal zone system ("Chicago 21, Ill."), so it can't be any earlier than 1943 (if Wikipedia is right, that is) and probably no later than circa 1960s (the ZIP code system replaced the postal zone system in 1963).

It appears that the padlocks that have "JUNKUNC BROS. SUCCR'S TO AMERICAN LOCK CO." on the front (instead of "JUNKUNC BROS. AMERICAN LOCK") are the earliest ones, as the company history on the official American Lock website (link) says "In 1919 his [John Junkunc's] company moved to larger facilities and acquired the American Lock Company, a manufacturer of cylinders."

I have a smaller Junkunc Bros. American Lock padlock with a shield logo, which I think may be the next logo used after the one on your NOS lock (interestingly, it also still says "PATS. PEND."). Its key is of the same design as the one for your lock, and it has a code on the top in the same format as the one on your brass lock, with a letter, a number, and another letter followed by 6 more numbers.