The mass equivalent of radiation is implicit in Poincaré’s formula for the momentum of radiation, published in 1900, and was used by Poincarén illustrating the application of his analysis. The equality of the mass equivalent of radiation to the mass lost by a radiating body is derivable from Poincaré’s momentum of radiation (1900) and his principle of relativity (1904). The reasoning in Einstein’s 1905 derivation. questioned by Planck, is defective. He did not derive the mass-energy relation.
HERBERT E. IVES, "Derivation of the Mass-Energy Relation," J. Opt. Soc. Am. 42, 540-543 (1952)