I wonder what STI was thinking? Also STI might not be Ruger, Colt or S&W but even I've heard of the company so they are kind of big...I wonder why they would do something so odd.
It turns out the gun is a custom build on an STI 2011 frame. The barrel is made by Schuemann. The barrel looks something like this:
I believe the slide has to be custom machined to fit the barrel and then fitted to the frame. The sight is fixed to the barrel itself, not on the slide. It's a weird 45, but its super accurate and fun as heck to shoot. I think it's labeled ".450" because it is made short-chambered, which leaves it up to the custom shop to more precisely finish the chamber. I didn't have the gun made; bought it at an auction, so I'm saying this based on what I've been able to learn about the gun.