If it is the same 'problem' as the TRF201, it is that the spur wasn't held held perfectly flat when tightened (then the plates are usually flat on there but the spur is not).
Just loosen it a bit until you can straighten it out by pushing the spur into a position where it doesn't wobble anymore. When this is the case, put even pressure on the spur plates and retighten it (looking once or twice in the process if it's staying straight - if not, try to straighten it with the setting you tightened it too and continue - if not possible, loosen it a bit and try again).
Sounds ridiculous? I have two of these gearboxes and they show the same behaviour - both with Tamiya and AE spur gears
