I had also heard about re-formed Krag brass being involved.  That makes no sense to me.  Quite good LN ammo can be made from .220 Swift brass by turning off the rim flush with the base and expanding the neck.  Or get a set of case forming dies and use NEW .30-06 brass.  Need a sturdy press for that.

Looking at the LN bolt from the back, with the bolt in place and in firing position, on an unmodified bolt, the end of the firing pin assembly is easily seen.  The mod is a heavyish piece of  sheet metal that is somehow fastened to the back of the bolt. Maybe with screws? Or maybe fits into grooves cut into the bolt.  The metal piece is blued.  If there were some sort of case failure with hot gas, the mod should deflect the gas and retain a broken striker.

IIRC, the bolt stop & release is just a plunger on the left side, at the rear of the action.  Push it down & pull out the bolt.