There was a change of part number for the detent block fairly early in 1500 production.

1G3685 Block - shaft locating
11G3187 Block - shaft locating - Commencing (G) A916 (and all Twin Cam)

The lower alignment pin was moved closer to the reverse shift rod. The detent block must be mated to the correct front housing.

There was also 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.