Hello Mr.2. I have never seen or read a factory directive on the hammer change to the solid configuration. I have read that their were some changes at A180000. Was the hammer one of them? Not a clue. I have observed that the hammers after the above Sn. were solid. Mean anything? Again not a clue. The factory did not waste good parts. Maybe a box of hammers showed up once the hammers with no holes were being used. Bingo, you have a batch with holes. Who knows?
By the way, what was the function of the hole in the hammer? go shoot.