Originally Posted by zaitsev44
That is a M1 carbine rear sight. My guess would be that the receiver was damaged, maybe a broken sight ear on the original receiver. Someone got the bright idea of grinding the remains of the sight ears down so they would be out of the way of the adjustment knob of the carbine sight. You can still see raised remains of the old sight ears as kind of low walls. My guess would be that a dremel or other cutoff wheel was used and then ground down. Then touched up with cold bluing. Drilling and tapping the receiver was probably how the carbine sight base was mounted. Also the receiver markings are not very clear, this receiver may have been rejected at armory level.
Do a google search of M1 Garand receiver images. It will show you what they should look like. And it will become obvious that the original milled sight ears are no longer there, and since they were originally milled out of the same single piece of metal, they can't just be put back on.
That receiver is complete poop. No nice way to say it. If you want to expose the seller some more, or at least the level if stupidity of the person who originally did this if it isn't the seller, ask for better pictures of the receiver markings, barrel markings, etc. to show that it is an IH receiver.