  We have derived new upper limits on several products (two at a time) of
lepton number violating $\lambda'$-type couplings from the consideration of
$\Delta S = 2$ and $\Delta B = 2$ box graphs. Each box contains one scalar
lepton and one W-boson or one charged-Higgs-boson as internal lines. Most of
these bounds are more stringent than previously obtained. Some of these product
couplings drive $K_L$ (and some other $B_d$) decays to two charged leptons at
enhanced rates. Some of them can explain the rare $K^+ \to \pi^+ \nu\bar{\nu}$
event recently observed at BNL.
