I dunno about a servo/motor interface being legal, ATF went to and fro over the Akins Accelerator in the mid 2000s before they decided that it is a machine gun because it added springs to provide the reset - thus in their view it became integral to the gun like a drop in auto sear, and falls under the ‘single function’ test.

The term shall also include the frame or receiver of any such weapon, any part designed and intended solely and exclusively, or combination of parts designed and intended, for use in converting a weapon into a machine gun