  It is well known that most actinides fission into fragments of unequal size.
The first attempt to understand this difference suggested that division leading
to one of the fragments being near doubly magic $^{132}$Sn is favored by gain
in binding energy. After the Strutinsky shell-correction method was developed
an alternative idea that gained popularity was that the fission saddle might be
lower for mass-asymmetric shapes and that this asymmetry was preserved until
scission. Recently it was observed [Phys. Rev. Lett. {\bf 105} (2010) 252502]
that $^{180}$Hg preferentially fissions asymmetrically in contradiction to the
fragment-magic-shell expectation which suggested symmetric division peaked
around $^{90}$Zr, with its magic neutron number $N=50$, so it was presented as
a "new type of asymmetric fission". However, in a paper [Phys. Lett. 34B (1971)
349] a "simple" microscopic mechanism behind the asymmetry of the actinide
fission saddle points was proposed to be related the coupling between levels of
type [40$\Lambda\Omega$] and [51$\Lambda\Omega$]. The paper then generalizes
this idea and made the remarkable prediction that analogous features could
exist in other regions. In particular it was proposed that in the rare-earth
region couplings between levels of type [30$\Lambda\Omega$] and
[41$\Lambda\Omega$] would favor mass-asymmetric outer saddle shapes. In this
picture the asymmetry of $^{180}$Hg is not a "new type of asymmetric fission"
but of analogous origin as the asymmetry of actinide fission. This prediction
has never been cited in the discussion of the recently observed fission
asymmetries in the "new region of asymmetry", in nuclear physics also referred
to as the rare-earth region. We show by detailed analysis that the mechanism of
the saddle asymmetry in the sub-Pb region is indeed the one predicted half a
century ago.
