In a substitution cipher, 16 letters in the English alphabet were used, and 10 letters were not used. In the puzzle, only 1 letter is left unsubstituted but it appears twice in the puzzle in the last two "words" of a cryptoquote and it is ambiguous because it refers to someone's name.

ABCDE = filled
FG = filled
H = filled
- = blank

So isn't there just 10 permutations for the "key" to this encryption? Someone said it was 10! but I can't wrap my head around that reasoning. Why? Who's right?