The milling cuts on the rear bridge appeared on late Remington M1903 receivers when there was trouble in developing the 03-A3 rear sight. In order to meet production schedules, Remington starting making the M1903 again, and they used the receivers that had already been milled for the 03-A3 sight ( actually for the 03-A3 sight that they had trouble with ). At some point, Remington M1903-A3 rifles can be found with LOWER serial numbers than Remington M1903 rifles. Must have been a confusing time.