I would have to disagree with you about the paint job being the only difference. Having both guns and having pulled them both apart to the last pin, spring screw I can say that the SIG version has a higher "finish" to every component. By finish in this instance I mean the quality of the machining and deburring and polish of the metal components. I don't disagree that the guns are the same, I do believe that the SIG is built to a better manufacturing standard and the way the two guns operate would indicate this as well.
And for sure I agree with you that the paint finish on the two guns is different!
BUUUTTTT.....My GSG runs 100% with Blazer bulk ammo....has from day 1. Maybe they made a mistake and stuck SIG parts in mine......lol
Unless I can see some pics of everything side by side I gotta call BS. Maybe yours was made on a Wed at GSG, and your Dad's on a Friday....