I believe it is some sort of computer problem handling the possessive “s'” See “ twenty day &#38 rations”

Do you see this same omission in your other examples?

Here is the document as found in the OR

HEADQUARTER DEPARTMENT OF THE GULF,

Alexandria, May 7, 1864.

Colonel E. G. BECKWITH:

COLONEL: The commanding general desires me to say that he intends to move on Simsport at an early moment. He directs that you send twenty days' rations for 40,000 men to the vicinity of that place, to meet the army on its arrival. Colonel Holabird has been directed to lend every aid in his power. You will understand that this emergency is pressing, from the time we have been cut off.

W. DWIGHT,
Brigadier-General and Chief of Staff.

OR V34 pt. 3 p. 493