Pays to go back and read books you haven't looked at in a long time.
Found an explanation why the front & rear turrets were changed.
So this became the beginning of the KI-67Ia. Model.
So when they changed out all the 7.9mm guns to 12.7mm and added the extra 12.7mm to the rear turret then this became the KI-67Ib. Model.
This part I did not know though at some point the 20mm cannon in the dorsal turret on the KI-67Ib was changed out for 2 x 12.7mm's. Clearly this must have been a better option than the 20mm cannon?
Also the only model to carry torpedoes was the KI-67Ia yet it only had a single 12.7mm gun in the tail.
AHIII KI-67 has torpedoes as well as the twin 12.7mm guns in the tail. Yet no KI-67Ia variant had the twin guns in the rear turret only the KI-67Ib.
So it looks as if we have a bit of a hybrid in game.