This isn't a WW2 K98k. The receiver is clearly a Czech post war receiver, not Mauser Oberndorf and the barrel is post war EG. There are some WW2 parts like the byf trigger guard which is a very late piece made at Mauser Oberndorf because it doesn't have provisions for the locking screws. It would never have been numbered originally so it was easy for the EG or Czechs to add the serial at the time this rifle was built/refurbed.
So they would of used a "fresh" receiver? What could explain the old serial number that they tried to ground off? Just a left over part?