Suppose I have two large photos, and show them to you one at a time. The first is the Budweiser beer wagon, bright paint, brass trim, Percheron draft horses all brushed out and showy, and then I show your the second photo, this photo of six mules hitched to a commonplace Army supply wagon, and I comment that this mule hitch is the "crack team" of this unit.
I also suggest that perhaps the "X" is not actually the wagon, but a mark placed on the photo to mark the spot where that named person is - note the pencil on the back of the photo shows an "X Capt. S H Manning"
Maybe not.
but I do think the "crack team" remark is meant to poke fun at these commonplace mules; mules don't get credit for their many good qualities.