How do you find the Federal Reserve district on a US 2 dollar bill?

Modern US $1 and $2 bills still carry the old-style Federal Reserve seal that includes the district letter and name. It's the large circle on the left side of the bill's front.

However older $1 bills (before 1963) and $2 bills (before 1976) weren't issued by the Federal Reserve System so they don't have any seal or district identifiers.