It is not necessary for the recipient of the letter to know who typed the letter. Mainly for the purpose of the sender, the typist's initials are indicated below the name and signature of the person sending the letter.
So if Jane Doe dictated the letter and Sara Smith typed it, you will see something like the below.
Sincerely,
JANE DOE (signature)
Jane Doe
President, Marketing
JD/ss
(All you see is the lower case initials of Sara Smith.)