There was a change (reduction) of depth of the drill hole holding the detent spring for 3-4 shift rod. This happened at with the "1600" type gearbox, beginning of 15GD power unit in January 1959. Also including slight increase of depth of the detent notch on the 3-4 shift rod, reducing tendency to pop out of 3rd gear. This change is recommended to retrofit (modify) all earlier 1500 type gearboxes. Find this information in Section F.1 of the Workshop Manual.

The shift rods changed multiple times, even reverting back to earlier design. Flats on the shafts would be for air vent to prevent motion resistance during shifting. Later gearboxes have a vent worm-hole in the front cover, then not needing flats on the shafts.

Shaft, 1-2 speed fork
  11G3079 (early MGA and all Twin Cam)
  1H3134   Com.(G) A828
  11G3079 Com.(G) A8011 (reverting back to first design)

Shaft, 3/4 speed fork
  11G3692
  1H3140   Com.(G) A290 (and all Twin Cam)

Shaft, reverse fork
  11G3080
  11G3137 Com.(G) A290 (and all Twin Cam)
  1H3133   Com.(G) A828
  11G3137 Com.(G) A8011 (reverting back to second design)