GSG 1911 Interchangeable parts
Okay here is the official list straight from the horses mouth (thanks Christopher):
Front & Rear Site
the Whole Main Spring Housing
Grip Panels (but not the screws because they're in metric)
These were all confirmed by ATI themselves, one note on the grip panel screws, it has been reported that you can change the bushings w/ no problem then use regular 1911 grip panel screws.