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"A little known fact about Einstein's September 1905 paper is that he didn't actually write E = mc²; he wrote the mathematically equivalent (though less euphonious) m = E/c², placing greater emphasis on creating mass from energy (as in the joust) than on creating energy from mass (as in nuclear weapons and power stations).

He editorializes too much. The title of the paper is, "Does the Inertia of a Body Depend Upon its Energy Content?". What he did was to calculate the energy difference measured for a body emitting radiaiton, in the stationary frame of the emitter, and one moving paralell to it's reference frame.

The result from the paper is:
K₀ - K₁ = 1/2*L/c²*v².

Ks and Ls are energy. His conclusion was,
"If a body gives off the energy L in the form of radiation, it's mass diminishes by L/c²." His stated equivalent was, "radiation conveys inertia between emitting and absorbing bodies."

The more general conclusion stated was,
"The mass of a body is a measure of its energy content."

"If the energy changes by L, the mass of the body changes by L/(9*10²⁰), the energy being measured in ergs, and the mass in grammes."

His concern in this was with knowing the physics of the world, not with creating anything. To test the theory, he proposed, "It is not impossible that with bodies whose energy content is variable to a high degree(e.g. with radium salts) the theory may successfully put to the test."