R K and R D ( * ) are two B-decay measurements that presently exhibit discrepancies with the SM. Recently, using an effective field theory approach, it was demonstrated that a new-physics model can simultaneously explain both the R K and R D ( * ) puzzles. There are two UV completions that can give rise to the effective Lagrangian: (i) V B: a vector boson that transforms as an SU(2) L triplet, as in the SM, (ii) U 1 : an SU(2) L -singlet vector leptoquark. In this paper, we examine these models individually. A key point is that V B contributes to B 0 s -B 0 s mixing and τ → 3µ, while U 1 does not. We show that, when constraints from these processes are taken into account, the V B model is just barely viable. It predicts B(τ − → µ − µ + µ − ) 2.1 × 10 −8 . This is measurable at Belle II and LHCb, and therefore constitutes a smoking-gun signal of V B. For U 1 , there are several observables that may point to this model. Perhaps the most interesting is the lepton-flavor-violating decay Υ(3S) → µτ , which has previously been overlooked in the literature. U 1 predicts B(Υ(3S) → µτ )| max = 8.0 × 10 −7 . Thus, if a large value of B(Υ(3S) → µτ ) is observedand this should be measurable at Belle II -the U 1 model would be indicated.