On the XX4-WE at least, the chassis came from the factory clearanced around the base of the spur gear. This was to help stop the spur touching the chassis on hard landings - changing one trackside isn't fun.
If it's a relatively neat looking job then it's quite likely factory. This pic from
Jimmy's XX4 review just about shows it:
Which spur are you running? I've always swapped the kit 84T item (purple) for the 82T (yellow) one.