Good question.
Maybe Samuel Wylie Crawford?
But since the “Richmond Whig” got the letter in December of 1860, that would be classified information, and would require a security clearance and a need to know.
5.56mm
I looked up Crawford. He was surgeon of Anderson’s detachment. So the memo could have been a note to a subordinate friend. That doesn’t explain how the Richmond Whig got hold of it in time to reprint 10 days later.
As a side note, Crawford will be in Sumter during the opening of hostilities and will be an infantry officer present at Lee’s surrender at Appomattox in 1865.
https://en.wikipedia.org/wiki/Samuel_W._Crawford