Prior to the outbreak of WW2 and the early stages of it the UK government set up companies in the USA to manufacture munitions for UK armed forces. These were the for runners of the later “Shadow factories”.
On Paper they were American companies, but in fact UK owned. As The USA was not officially on any one side at this stage a lot of the munitions made did not carry any markings.
Bofars and Oerlikon also did this as well, as many forces used the famous 40/60 and 20mm gun, so the munitions were mass made and shipped out. To avoid any mix up of who was buying the rounds they too were not marked. They should have been stamped when at destination.
As the war progressed, The Allies set up factories and munitions were manufacture in mass. To cut cost, expensive metals like copper were replaced with other materials.
On Naval, both UK and USA the AA shells, from 3”. 4.5” and 4.7” had the copper or steel Firing vent replaced with resin impregnated cardboard. Like the material Mustang and Spitfire drop tanks.