  In a companion paper (Paper I: Jones et al. 1996) we report the discovery of
a cosmic microwave background decrement, indicative of a distant cluster with
mass ~10^{15} solar masses, towards the quasar pair PC1643+4631A&B
(z=3.79,3.83, separation 198''). To search for the cluster responsible, we have
obtained R-, J- and K-band images of the field and have also carried out
optical spectroscopy of selected objects in it. No such cluster is evident in
these images. Assuming the cluster causing the decrement is similar to massive
clusters already known, our magnitude limits imply that it must lie about or
beyond z=1. This provides independent support for the X-ray-based distance
argument of Paper I. The cluster must gravitationally lens objects behind it;
for a cluster z around 1-2, the Einstein-ring radius for sources at z ~= 3.8 is
~ 100''. Simple modelling, producing simultaneously the S-Z effect and the
lensing, shows that the source positions of quasars A and B lie within ~10'' of
each other and may indeed be coincident. The two quasar spectra are found to be
remarkably similar apart from their one-percent redshift difference. Assuming A
and B are images of a single quasar, we present a possible explanation of this
difference.
