Shell marked on bottom:
40mm MK2 44
Welcome here R8chel!
This is a US made case, originally in the caliber 40x311R 40/L Bofors.
At some point after it’s production (probably even right after in case it failed QA) it got converted into a saluting (blank) round.
Thank you. That does help.
Looks like remnants of 1944 original manufacturing date for the unmodified case.
40mm saluting guns are widely used by the Navy aboard larger ships and on some shore bases. They would have used cases on hand, or from rounds which had been downloaded (age, etc), and modified them to saluting cases. The loading info was usually stenciled on the side of the case with 2 or 3 letters to identify the facility, a 2 digit year, and a lot number.
Don’t know what the inside of this 40mm looks like but if your going to use any type of an enclosure to put a AC power cord through, you must use the “Underwriters Knot” to secure the cord from being yanked out and posing a electrical hazard. Remember that if you decide to build a neat lamp.