Several features of an analytic (infinite-dimensional) Grassmannian of (commensurable) subspaces of a Hilbert space were developed in the context of integrable PDEs (KP hierarchy). We extended some of those features when polarized separable Hilbert spaces are generalized to a class of polarized Hilbert modules, in particular the Baker and τ -functions, which become operator-valued. Following from Part I we produce a pre-determinant structure for a class of τ -functions defined in the setting of the similarity class of projections of a certain Banach *algebra. This structure is explicitly derived from the transition map of a corresponding principal bundle. The determinant of this map gives a generalized, operator-valued τ -function that takes values in a commutative C*-algebra. We extend to this setting the operator cross-ratio which had been used to produce the scalar-valued τ -function, as well as the associated notion of a Schwarzian derivative along curves inside the space of similarity classes. We link directly this cross-ratio with Fay's trisecant identity for the τ -function (equivalent to the KP hierarchy). By * Partial research support under grant NSF-DMS-0808708 is very gratefully acknowledged.