Dave - To the best of my knowledge and belief, the shaft that runs through the limber (indicated in yellow on your drawing) is called a "limber pole."
cf. James C. Halett, Edwin Olmstead, and M. Hume Parks,
Field Artillery Weapons of the Civil War, Revised Edition, Newark: University of Delaware Press, 1988, pp. 218-19 and
https://civilwartalk.com/threads/lim...r-away.142610/
Don