Hi,
The screw enters on the side with the short diff outdrive.
In the short diff outdrive is the thrust race, in the long diff outdive is the spring and the long nut.
This is the exploded diagram for the car,
http://www.hbeurope.com/exploded/61700.pdf.
Details of both types of diff are there.
The other difference between the old and new diffs is the size of the trust race balls. The WCE uses 8 x 1/16" balls rather than the 2mm balls in the original diff. This is due to the wider diff screw taking up more space.