Quick shift will not fit my 1600e gear box

I have purchased a "used" quick shift gear stick apparently off a ford mexico single rail 4speed box to fit it to my 1600e single rail 4 speed box and although the fork "looks" the same and the fixing screw thread is the same ,the fork just will not locate in to the sellector.
I have measured the fork dimensions and the inner width ( which locates with the sellector rod) is the same , the width of the 2 rounded bits on the fork at there widest point is very close, existing stick is 0.2mm smaller . Is this enough to stop the fork fitting the sellector rod.

Picture shows the existing stick and the replacement quick shifter




Before I get the grinder out is there anything I could be doing wrong as I thought all 4 speed gear sticks fit all 4 speed gear boxes?