WW1 'Dummy' cartidges for U.S. Navy 6-pounder?

The shell casings seen in the photo are from the site of the July 10, 1926 explosion of the U.S. Navy munitions depot at Lake Denmark, NJ. The casings have no markings and I believe they are for a U.S. Navy 6-pounder. They’re short and appear to have a brass plug in the base instead of a primer. An inside view shows lack of a raised center area around where the primer would be. The shell bottom of a 6-pounder seen in the photo is for comparison.