I would say it is a difference in the gear box design which means it needs the extra spacer.
Trust me you can't run an x2c/mad monkey without that spacer in there because as soon as you slightly tighten the slipper it binds up the gearbox.
The gearbox screws for the madrat and the monkey are different length also, the ones on the rat are 2mm shorter.
I think the motorplate is spaced further away on the mid cars than they are on the rear mounted cars, and as the layshaft is the same on both cars the slipper plates go further into the gearbox housing on the mid motored cars than they do on the it needs the spacer to keep the slipper plate on the bearing. On the rear mounted cars the flat in the layshaft stops the slipper plate going in to far and so keeps it away from the bearing.
I guess it was cheaper to put a small spacer in there than to redesign a layshaft
