I recently started collecting WW2 militaria, and I got a 20mm shell for Christmas. Total length is 185mm, excluding the projectile its 110mm. It says “NECO 1942 20MM M21” on the headstamp, and some other text I can’t read or get a good photo of around the tip of the projectile. My own research on the headstamp tells me its for a Hispano autocannon and was made in Wisconsin.
My main question is, what kind of shell was this originally? Was it HE, AP, or something else entirely? I’d appreciate anything else you could tell me about it though.