what is the name?

[Larz60+]

The characters corresponded to the decimal position in the alphabet:

correct, as stated earlier:

the value was arrived at by adding the letter position value to 0, thus 'M' being 12th character since A is 0,

which is also ASCII 12 (decimal)

I found an interesting pdf about the teletype history (invented in 1907) that was written for the 50'th anniversary.

Even though I started programming in the 1960's, I never used one as computer peripheral until the late 70's or early 80's. But everyone had one, which was usually put next to the poor receptionist or telephone operator because they were so noisy! But, I guess you could say that the telex was an early form of email.

One (perhaps last) comment, note page 37:

Since the stunt, box can recognize sequences of control codes as well as single codes, the number
of permutations available for a coding system runs into the
thousands.

Thus the reason for the ctrl key on our keyboards today.
literally. You literally held the control key down while typing the second letter.

Here's an image of the code wheel (the beginnings of the ASCII (coined in 1963) character set)