If we're thinking about the stonecutter for the marker as a transcriber, yes, transcription error is likely. The question should be, who gave the stonecutter the veteran's unit designation, and how did they know? It was probably a family member who may not have been sure about it.
It's unlikely the error on the tombstone (should be Company B) was due to anyone misreading a document. Except in rare cases, muster rolls and other wartime records would have been unavailable. At the time Confederate records belonged to the War Department, and most inquiries into them concerned eligibility for pensions. These took weeks if not months to forward, process and return.
In all cases the War Department retained original rolls, the photocopy method was yet to be developed, so the only way anyone from Mississippi would have seen the records would be for them to visit Washington and get permission to see the actual records. That's not easy to do even today.