I cannot see how that would be a mistake by the machinist, I would know, I'm a lathe guy myself. The finish on the chamfer on the back is definately too nice to be accidental. Besides, you think it would be noticed since that would be where the front sight blade would normally go? That undercut has to have some sort of use...
Oh, and round stuff is turned, not "lathed", sorry, pet peeve of mine... No offence Nine.