Einstein's second mass-energy formula m'/m = e/c^2

In "Out of My Later Years", Einstein's introduces another
mass-energy equivalence formula after kinetic terms.
So if it's sort of Einstein's second-most famous formula,
why hasn't anybody heard of it?
m'/m = e/c^2
It introduces that the terms in the rotational, make
for that mass-energy equivalence only sits in the
rotational setting, among all the other usual terms.
It's introduced in a brief note near the end of
the material on science in Einstein's "Out of My
Later Years".
It really makes for a sort of way to make it so
that the space-contraction results real while
also that the linear is rather Galilean, while
still fulfilling all the usual derivations, if
not necessarily the rhetoric or intuitions,
yet very intuitionistically while all formally.
It's pretty great I wonder why it's not well-known.