  Supported only in the two 1905 Einstein's papers on Relativity and a very
rigid respect for the historical context, an analysis is done of the derivation
of the universal mass-energy relationship. It is found, contrary to the today
accepted Physics knowledge, that a body's Rest Mass measures its Potential
Energy in the 1905 context. After emphasizing the difference between 1905
Relativity (1905R) and Special Relativity (SR), the developing of a 1905R
relativistic gravity is started for a small mass m material point moving in the
central gravitational field of a great mass M one. A formula for the rest mass
m_0 as a function of its distance r from M is obtained. Finally, those results
are applied to an atomic clock in a gravitational field, reaching a factor to
obtain the clock time rate change very close to the GR one. The factors from
1905R and GR are compared, emphasizing the absent of a singularity in 1905R. In
the conclusions, a new road for the development of a 1905R relativistic
mechanics is declared, related with the discovery that Rest Mass measures
Potential Energy, done by 1905 Einstein even if not realizing it.
