 In 1971, one of the few engineering changes ever made to the Standard model took place when the original 22-year-old receiver forming dies wore out.As a precursor to changes to come with the 1982 introduction of the MK II series, the slot for the magazine follower extension on the grip frame was moved from the right to the left side.Designated the “A 100” frame modification, this alteration facilitated the eventual improvement of the Standard pistol by the addition of a bolt hold open device as part of the eventual MK II upgrade.