A model for a quantum-mechanical weak link between two superconductors is discussed in which the tunneling of electron pairs is described by ``isotopic spin'' operators (${\mathit{T}}_{1}$,${\mathit{T}}_{2}$,${\mathit{T}}_{3}$) obeying [${\mathit{T}}_{\mathit{a}}$,${\mathit{T}}_{\mathit{b}}$]=i${\mathrm{\ensuremath{\epsilon}}}_{\mathit{a}\mathit{b}\mathit{c}}$${\mathit{T}}_{\mathit{c}}$. In the thermodynamic limit of infinite size for the superconductors, the model reduces to the quantum ``Josephson pendulum.'' However, the model also holds true for mesoscopic grains, and thus allows for the quantum description of lowest-order finite-size effects.