On the right is Exakta mount.

Of the two M42 conversions, I prefer the lens in the middle.

The the Exakta mount bayonet flange remains on the lens to the left, but there is no point to that if 0.74mm has been milled off the mount surface to match M42. The Exakta flange prevents an M42-to-SA(Sigma) adapter from screwing down all the way to the mount surface.

The M42 conversion in the middle is extremely professional. It's perfect. It might have been a factory option.