There are also two cuts either side of six o'clock that act as feed ramps. Your description of it looking like a rat chewed it may be from removed rust. It may also bee the original owner dropped the barrel on that thin cutout area before installing it (it bends easily when outside the reciever), and had to break it off to keep it from interfering with feeding.
Gunsite Orange Hat Family Member
CMP Certified GSM Master Instructor
NRA Certified Rifle Instructor
NRA Patron Member