Two motors off one line are not the same motor.
A body that can be stolen, altered or substituted needs some way to show it is the body that was certified, and software cannot supply one because software is exactly what an adversary replaces. Manufacturing variance is the classical answer, and silicon has been using it for years. This topic asks whether a drivetrain carries the same thing, and prices the answer in bits, because the price is what decides whether it is useful.
The signal is real: about 21.5% unit-to-unit spread in the coefficient that could serve as an identity, against a 2.94% resolution floor. Only one of the four per-joint coefficients is identifiable well enough to use, and it dissolves at a 34 °C temperature rise unless you measure the winding. But the constraint that decides the outcome is none of those. At roughly 2.8 bits per coefficient, a seven-joint arm carries about 13.5 bits and a full humanoid about 56. That is enough to check whether a unit is the one you enrolled. It is not a cryptographic key, and no realistic joint count makes it one.