A quick look at Grellman's book leads me to believe it's a N-15.
Description from book: "Heavy, curved line (top of a misplaced 5) at left top of 5 connecting it to the curl above."
So it's the remnants of a mispunched 5.
Description from book: "Heavy, curved line (top of a misplaced 5) at left top of 5 connecting it to the curl above."
So it's the remnants of a mispunched 5.





















