1. )
I am not a mate. I have never served on any ship.
2.)
2 and 3 are numbers but comparing numbers with letters is wrong. The equivalent of a number would be a word, since a word consists of at least one letter and a number consists of at least one digit.
3.)
Nevertheless, a letter is by definition a piece of type that prints a single character. This term is commonly used in printing-related speech but also in Information Technology.
4.)
I checked my keyboard, and it does make a difference. Of course, I do not know what kind of keyboard you use. The series 23., would look like "ยง:; when typed using my keyboard layout.
Like I said before - who are you trying to fool?