More Heinlein?

Pretty obscure…….

This story tells about the journey of a mechanical genius from his self-imposed exile from the rest of humanity to a more normal life, conquering the disease myasthenia gravis as well as his own contempt for humans in general.

Being born a weakling, he channeled his intellect, and his family’s money, into the development of the device patented as “Synchronous Reduplicating Pantograph”. Wearing a glove and harness, he could control a much more powerful mechanical hand simply by moving his hand and fingers. In the story, these devices became popularly known as “Xs”. In reference to this story, the real-life remote manipulators that were later developed also came to be called Xs. Specifically; an “X” is a device which is controlled by moving a model of the device; usually a pair of robotic hands that are controlled by sensors in a pair of gloves.

No relations to that other X, though .What is X?