I always put the flat on the flat of the barrel lug.
I think the whole design is stupid. It's the only gun I've seen where orientation on the barrel matters.Right, and thats what makes sense, but everything I've read online said that the rounded part needs to be on the barrel lug. Maybe Sig realized it made more sense to have the flat against the lug and changed it. Unfortunately, there is not much information in the manual that shows how to orient it.
I've only had experience with the one in the ops photo.That's incorrect on all but the more recent RSAs. The correct method has always been to keep the narrower part of the flange "on the cross axis and pointing at the slide rails". Think of it as providing clearance to the slide rails.