As we have seen in our previous article, Einstein published for the first time in 1905 a demonstration to try set the equivalence of inertia and energy, or mass and energy.
However, it is only in 1912 in the Manuscript of Special Relativity, that the famous formula E=mc2 appears in its well known form.
In this draft, however, the equation, which demonstrates that mass is a form of energy, appears in a somewhat different form. Originally, Einstein used the variable "L" for energy (the "L" stood for Lagrangian, a general form of energy); later, he crossed out the "L" and changed it to "E." The letter q stands for the velocity of the moving body.
In the below english translated extract corresponding to the above original manuscrit, we recognize the formula E=mc2, with m=γm0 (rest mass) as seen in our article Introduction to Four-momentum vector and E = mc2
Then Einstein makes the supposition that the body is moving with a velocity q<<c, and expands in a Taylors series
We recognize the equation we have exposed in our article Introduction to Four-momentum vector and E = mc2