  We prove that the tau-function of the integrable discrete sine-Gordon model
apart from the "standard" bilinar identities obeys a number of "non-standard"
ones. They can be combined into a bivector 3-dimensional difference equation
which is shown to contain Hirota's difference analogue of the sine-Gordon
equation and both auxiliary linear problems for it. We observe that this
equation is most naturally written in terms of the quantum R-matrix for the XXZ
spin chain and looks then like a relation of the "vertex-face correspondence"
type.
