It is a combo gun made from a Thompson M1A1 (top) and Remington 870 12 ga. (bottom). It was originally designed as a prop for the movie Aliens. There was a company that produced functional replicas. But Maltese would have to tell us if his is one of those or one he built/commissioned himself.
Here is a non-functioning replica that is built with authentic parts which details the components and construction process pretty nicely. http://www.tk560.com/m41a.html
Here is a non-functioning replica that is built with authentic parts which details the components and construction process pretty nicely. http://www.tk560.com/m41a.html