I pulled my SMC out of the gunsafe and examined the magazine. The profile is about the same but there is a clear magazine notch on the FRONT of the magazine, which would have been visible in your previous pictures if I had bothered to check before posting.
I did observe that the magazine was completely without markings, just like yours.
How do you know the magazine is for a .380 ACP gun? Could it be .32 ACP?