Some interesting information on mounted infantry and the carrying of equipment from:
Wilder's Lightning Brigade
by Glenn W. Sunderland
pp 194
"Our knapsack was cut to pieces and remodeled so as to fit closely and compactly to the cantle of our saddle; our haversack we have never carried, and it was fastened to the near side of the pommel; our canteen was hung to the other side of the pommel, as was also a good hatchet. When on a march our cartridge box was swung loosely to the pommel, and if we had five day's rations, it hung on the right side, and if our rations were out it hung on the left, but was never fast, so that any moment we could dismount and sling it over our shoulder. The muzzle of our gun, up to the first band, was stuck into a boot at the right side by the saddle girth, and a strap around our shoulder held it up by our side when riding, and this was the only article strapped or fastened to us. Under our saddle we carried two good sleeping blankets, a dog tent, and a gum blanket. In our cartridge box we carried 40 rounds of metallic cartridges, and in our saddle pockets 40 to 60 rounds more, an extra horse shoe, some nails, etc."
In regards to the Knapsack, we looked at several pictures of geared up Mounted Infantry mounts. We noticed the knapsack is lying just above the saddlebags. It appeared to us (remembering that these are grainy photo's that usually only show the mount from the front) that the knapsacks were double bag and were placed over the horse in an "open" position - i.e. laid out as if for inspection. We are not sure if they were placed to expose the flaps (inside of knapsack) to the outside of the horse or to the inside. We also cannot tell if the "pockets" of the knapsack were secured in a better manner since these pockets really depend on the knapsack being closed. I imagine that some modification was done to secure this. In MacGee's book he mentions the straps being cut up and modified to secure it better. He does not say if this was to secure it to the saddle better or to secure these pockets better.
Of course, we may be completely mistaken about the layout of the knapsack. It have been in a folded position - attached to the cantle. Please feel free to email us if anybody has any additional information on this!