When I first ran across this fact, I couldn't figure out what they meant. But it makes sense that the mittens that the soldiers wore would have to deal, somehow, with that trigger finger.
So a little online searching lead to this:
World Turn'd Upside Down if you want to read more about this -- with actual mittens knitted from this pattern.)
Would you like your own pair? Here are directions!
Peterson’s Magazine, February 1862, Vol. XLI, p. 176.
TO KNIT A MITTEN WITH ONE FINGER. – Cast on three needles sixty-four or more stitches according to the size desired, and knit about two inches of ribbing; then, at the middle of one of the needles, bring in the thread to make an eyelet to begin the widening for the thumb; then knit one round, knitting in that stitch; on the next round, make an eyelet on each side of the first one, and so on every second round, making the eyelet to the right or left of the previous one, widening until about seventeen holes are made on each row; then, take off all these extra stitches on a string, cast on five or six stitches and knit one round, narrow one stitch at each end of the cast-on stitches, and again at the second round; then, knit until time to make the finger, and take off on a string one-fourth of the stitches, dividing them equally on each side of a line with the thumb, cast on four or five stitches to make room between the fingers, knit one round, and narrow one at each end of the cast-on stitches, knit as long as you wish the mitt, then narrow and finish. Thumb – Put on the stitches from the string, fasten the thread at the right hand side, knit on until you come to the cast-on stitches, take up like for the heel of a stocking, knit one round; then narrow at each end of the cast-on stitches until the thumb is reduced to the size desired, knit until long enough and finish. Finger – Take up the stitches off the string, narrow one or more stitches, knit as long as the mitt.
Knit purlbus unum.