Trying to figure out what they did here. Does it look to you like they either ground or machined the top 180 degrees or so of the barrel deep enough to remove most of the rear sight pin hole? Looks to me like maybe they used a tool post grinder or put it between centers on a surface grinder and ground the top only. Does the back of the barrel measure bigger in diameter across than it does top to bottom? As if material was removed from the top only.

Thanks,
John