I have a 1939 J55 as well, also refinished. Could it be the same guitar... FON starts with an F so my theory (since the F means it was shipped in 1940, but the guitar has the 1939 specs, including stair step headstock and mustache bridge) is that the guitar was built in late 1939 and shipped in early 1940.
Such great and rare guitars. It’s great to hear of this one being rescued.
Mark