It is for a Traxxas micro servo, which as it turns out has a larger diameter gear on it. In between a standard servo and and micro. The Reefs servo has a standard sized micro gear on it.
Traxxas vs Reefs
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Standard servo vs standard micro
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So the stock Traxxas servo saver will not fit the Reef's servo. Maybe a shoft servo saver from Axial would work. Let me find the part number...