Thanks!
Yes, the L is always hidden. In case you missed it (though it sounds like you didn't), you can logically deduce it by using the L10 clue. You can also deduce it in puzzle 12 with no ambiguity. Early versions of the book had another hidden leftover in puzzle 5, but I removed it when remaking that puzzle. If I do another update, I might foreshadow the mechanic early on using a partially-covered leftover letter.