I think the tablet catchers were the same height on all the locos I think the issue was have been the height of the window , which on a 121 was much higher than the other locos , this may have made the placement and retrieval of the staff from the snatcher more difficult? So in the original postion the snatcher was sitting at three o’clock from the cab side but in the revised position was the at five o’clock from the cab side and when pull back was easier for the snatcher man to retrieve ?