Question concerning interior component of an Ammunition Chest

[sumdalus]

Specifically dealing with a Chest for a 12-Pounder Field Howitzer. (No others, please.)

According to the period "Instruction for Field Artillery", the ammunition chest for the 12-Pdr Field Howitzer has
"Twenty-eight props, for the upper tier of shells and spherical case, four in each division, except
for the canisters."

Can anyone clearly describe to me what a prop actually looks like or provide a drawing/photo of one?

Thank you for the help.

Dale Lee Sumner
"The Forgotten Marines"