Larry.
That looks like the ATI scope mount.
I have installed many of those for friends, but never on my rifles.
That third screw that you added really makes a difference in keeping them tight.
The pictures I submitted of the bolt modification were built in the late 70's.
The top two on the left started off as a D&T of sorts and are made out of 1/4" flat steel reshaped, and the one has the original Ball off the old bolt D&T'ed onto the shank.
Where I mounted the handle onto the bolt body, I did not countersink the screws.
Both parts are threaded, then put together and the screw shank is ground and polished flat.
The bottom two are welded on.
I did those after I taught myself to Gas Weld.