Affirmative on it being a Sigma. They were nice little guns. Designed as an "In your face" last line of defence gun. There wasn't a traditional mag release on these, the mag has tabs on the left and right side of the grip that you squeeze and pull the mag down.
It's a great gun, in my opinion, for a deep cover concealment. You could wear it with nothing on but a pair of boxers and no one would know.
It's a steal at this price... if you want a .380