In the 1905 paper about mass-energy equivalence, Einstein's final equation (original german) is:
K0-K1 = L/V2 v2/2
Where V is the speed of light, L the energy of both opposite light beams, and (K0-K1) the energy change at the system composed by
both frames of reference (considering one at rest with respect to the second one moving at v speed over x axis).
In this "newtonian" formula for Kinetic Energy, the mass came from the energy of both beams of light, each one having L/2 energy.
Einstein, as far as I know, never was able either to probe this approximation to the final statement (which leads to E0=m0c2), nor he could give a satisfactory explanation between the relationship between kinetic and electromagnetic energy in the same formula.
Does it mean that he was actually satisfied thinking that mass has an electromagnetic nature, as Poincarè thought with his cannon of light shots that had a kinetic recoil, or the true original nature of this (questionable) formula has been buried into oblivion by those who adopted his expression?
If this is so, then (as per Einstein's extension), the entire mass M of the point-like body can be decomposed into chunks of (L/V2) bits of matter until this mass M is entirely transformed into light. For this to happen, it's only needed to keep the "thought experiment" for enough time, until the mass M dissapear bit after bit while the light beam is turned on (what can take million of years?).
These two doubts (electromagnetic nature of mass and mass M consumed given enough time) arise when I analyze the original paper with utter care.
Where am I wrong? Please, refer only to the 1905 paper and not to changes that were developed since 1910 by other physicists.