What does the typist code “m” mean after the writer’s name? For instance:
BS/m
where BS is the name of the author of the letter
The initial of the secretary who took the letter in dictation and/or typed it.
I guess if there is a typo, the boss knows who to blame. 
But why would it only be one initial? Isn’t it usually two letters? That’s wht I was asking. I was wondering if “m” meant “me” or something like that. For instance, what would you type if you typed your own letter?
There wouldn’t be anything, because if you type your own letter, you don’t need to keep track of who dictated it versus who typed it.
As mentioned, it does seem that the first initials are the letter-writer’s initials (capped), while the second initial(s) belong to the typist:
http://www.metrokc.gov/recelec/archives/policies/inf141ae.htm
At least according to King County.
Why only one initial? Perhaps the typing pool is quite small in that office–you know to look for either (M)ary or (S)teven, because no one else would have typed it.
I heard once that if BS typed it, it would be: BS/hs
meaning BS by her/himself
And I agree with toadspittle about the small typing pool.