"War Date 15th Corps Badge and CDV of Soldier Wearing One: The badge is a very handsome example with red enamel for the 1st division. Fine condition but the attaching pin is gone from the back. Also included is a 31st Iowa CDV showing a soldier wearing precisely this type of 15th corps badge. The corps badge has a miniature cartridge box with the words “FORTY ROUNDS” above the US plate. As the legend goes the 15th corps adopted this design because in the earlier days of the war the 15th corps had no adopted insignia, and reportedly when one of the men was asked why he wore no corps insignia the hardened soldier responded by slapping his cartridge box and telling the neophyte that this was his corps badge. Whether true or not the story persists as part of the history of the 15th corps.'
This might possibly be the original story. The Soldier's Journal of April 20th 1864:
CORPS BADGES
A correspondent of the Chicago Tribune, writing from Huntsville, Alabama, says,
the troops which came here from the Army of the Potomac brought with them various ornamental habits and customs 'that were new ' to the Western soldiers. Among them was the corps badge which designated the corps to which officers and men were attached. For instance, the badge of the Eleventh Corps is a crescent, that of the Twelfth a star. The badge
is made of any material, gold, silver, or wool flannel, and is worn conspicuously on some part of the clothing. The Western corps have no such badge.
How an Irishman explained the matter is thus told :
A soldier came by the headquarters of Gen. Butterfield, a tired and weatherbeaten straggler, he was one of these, who made Sherman's march from Memphis to Chattanooga, then to Knoxville, and was now returning in the terrible cold of that returning march, thinly clad, one foot covered with a badly-worn army shoe, the other with a piece of raw hide bound with strings about a sockless foot both feet cut and bleeding. "Arms at will," he trudged past the headquarters' guard intent only upon overtaking his regiment.
"Halt," said a sentinel with a bright piece, clean uniform, and white gloves.
" What do you belong to ? "
" Eighth Misshory, sure."
"What division?".
" Morgan L. Smith's av coorse. "
What brigade ? "
" Giles Smith's Second Brigade, of the Second Division."
" But what army corps?"
"The Fifteenth, you fool. I am one of the heroes of Vicksburg. Anything more, Mr. Sentinel ? "
' Where is your badge ? "
" My badge, is it ; what is that ? "
"Do you see this star on my cap ? That is the badge of the Twelfth corps. That crescent on my partner's cap is the
badge of the Eleventh corps."
"I see now. That is how yez Potomick fellers gits home uv dark nights, ye take the moon and shtars with ye."
"But what is the badge of your corps ? "
Making a right about and slapping his cartridge box, our soldier replied :
"D'ye see that? A cartridge box with U. S. on a brassh plate and forty rounds in the box and sixty rounds in our pockets. That's the badge of the Fifteenth, that came from Vicksburg to help ye fight Chattanoogy!"
https://chroniclingamerica.loc.gov/...dges&andtext=&dateFilterType=yearRange&page=4