The lanyard ring uses a cross pin to hold it in place. If you remove the grips and look at the side of the grip frame near the lanyard ring recess, you will see a small hole going through the bottom of the grip frame. A simple pin will hold the ring in. These can often be made from a broken drill bit.
To remove the filler in the recess, it really depends on what the filler is. If it was welded up, you are basically out of luck, unless you want to drill out a new recess. I think I would drill a small hole in the center of the filler to determine exactly what it is, and then perhaps screw a screw eye into the hole and see if I could pop it out if it looked like I could accomplish it that way.
Trying to undo what someone else did years ago can be an exasperating job.