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.
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