  Unique among alkali-doped $\textit {A}$$_3$C$_{60}$ fullerene compounds, the
A15 and fcc forms of Cs$_3$C$_{60}$ exhibit superconducting states varying
under hydrostatic pressure with highest transition temperatures at $T_\textrm
{C}$$^\textrm {meas}$ = 38.3 and 35.2 K, respectively. Herein it is argued that
these two compounds under pressure represent the optimal materials of the
$\textit {A}$$_3$C$_{60}$ family, and that the C$_{60}$-associated
superconductivity is mediated through Coulombic interactions with charges on
the alkalis. A derivation of the interlayer Coulombic pairing model of
high-$T_\textrm {C}$ superconductivity employing non-planar geometry is
introduced, generalizing the picture of two interacting layers to an
interaction between charge reservoirs located on the C$_{60}$ and alkali ions.
The optimal transition temperature follows the algebraic expression, $T_\textrm
{C0}$ = (12.474 nm$^2$ K)/$\ell$${\zeta}$, where $\ell$ relates to the mean
spacing between interacting surface charges on the C$_{60}$ and ${\zeta}$ is
the average radial distance between the C$_{60}$ surface and the neighboring Cs
ions. Values of $T_\textrm {C0}$ for the measured cation stoichiometries of
Cs$_{3-\textrm{x}}$C$_{60}$ with x $\approx$ 0 are found to be 38.19 and 36.88
K for the A15 and fcc forms, respectively, with the dichotomy in transition
temperature reflecting the larger ${\zeta}$ and structural disorder in the fcc
form. In the A15 form, modeled interacting charges and Coulomb potential
e$^2$/${\zeta}$ are shown to agree quantitatively with findings from
nuclear-spin relaxation and mid-infrared optical conductivity. In the fcc form,
suppression of $T_\textrm {C}$$^\textrm {meas}$ below $T_\textrm {C0}$ is
ascribed to native structural disorder. Phononic effects in conjunction with
Coulombic pairing are discussed.
