This movie was big on recycling other stuff so I imagine the main body is made from something else and then stuff stuck onto it. Perhaps a toy gun of the era.
I am already entertaining the idea that maybe some of the detail is actually melted and smooshed to make it less recognizable so as far as that goes I think you are not off the mark psicorp.
The closest base pistol I can think of is the Walther PP. That's assuming that the 'real' barrel ends well back from the sight post and that they have built up the rear of the weapon to shroud the hammer action. Of course it may not be moulded off a real world weapon at all. It could just as easily be that they kit bashed a BB gun or water pistol.
There's a vintage toy cap gun In the junk yard that I almost thought could have been the base gun, the top body? car part or refrigerator part. looks like that bolt Is some kinda ground connector. just trying to help. spiting out Ideas...
Maybe a hot glue gun. Many have the same angled back end. The one shown isn't one I think they used, I am using it to illustrate my theory. The glue gun shown has the side of the handle and body on the same plane and I have found many other conventional ones that are like that so perhaps that is what was used.