I believe the most significant number would be one positioned with the 'crossed-rifles'.

That would be the U.S. Infantry regiment's number.

On SAW era cap badges, the regiment number is above the 'crossed rifles' and the company letter, below.

However, in this photo of stenciled haversacks, I cannot tell if it is a number or letter, below the crossed rifles.

(The numbers at the bottom of the bag, below the 'U.S.', may be a rack or I.D. number).

Attached unrelated photo shows the layout used on period cap badges.

---