Wow I would love to see the other 199 images in that group! Are you going to post them?

This looks like it was taken during a training exercise. Everyone is nice and uniform in their rain gear and no one is carrying a weapon. The guy in the foreground is screwing a fuse onto that shell, which is non-fixed, i.e. does not use a brass casing. After being fused, these shells are rammed into the tube, followed by whatever bagged powder charge the fire direction men deem adequate to reach the target. The breech is then locked and a primer--which looks like a rifle cartridge minus the bullet--is inserted in the breach. The pulled lanyard fires the primer which ignites the powder bags in the breech, sending the shell on its way. Any excess powder bags will be burned when the shoot is over. The shells look to be in the 150mm range.

I find the wicker baskets these were shipped in to be interesting. Ours came either in wooden crates or they were shipped pointing upright on a forklift pallet, but I served some 50 years after this was taken.