I picked up a new grip. Went to put it on and see there is a flat blade head in the bottom of the grip. Looked at it from the top through the receiver and there's another screw head but it looks like a square insert? Hmm...a little weird. So what's the proper way to remove it? I don't like surprises. TIA.

Take the receiver cover off, and remove the spring, and bolt/carrier assembly.

Then, unscrew it from the bottom.  There will be a "T" nut loose in the receiver.  Replace grip... making sure you orient the T nut correctly.  You will see.

This aint rocket science.  Just do it.

The reason for the square hole in the end of the pistol grip screw is so it could staked in place keeping the pistol grip from becoming loose from the vibration of firing. A square punch was inserted in the screw once the pistol grip screw was tightened and hit with a hammer so the threads would spread out locking the screw in the nut. The screw will be hard to turn but will come out and seeing how you are not going to be going into combat firing full auto there is no need to restake the screw.