On the face of a dollar what does the letter L inside the circular seal mean?

Answer 1

It indicates which Federal Reserve district that requested the bill.

There are 12 districts with letters from A (Boston) to L (San Francisco). When retail banks in a district need additional bills, their request is sent to the Federal Reserve bank in the district where they're located. That district then places a request with the Treasury to supply new bills.

There's a common misconception that each district prints its own bills, but that's not the case. All bills are printed by the Bureau of Engraving and Printing at facilities in Washington and Fort Worth. Each order is coded with the the letter of the ordering district.