This is another cult lens, a standard lens made for the older M42 screw mount Fuji SLR cameras. There is a newer 55mm f1.6 X-Fujinon that is a different lens, which does not have the unique bokeh exhibited by the M42 version. Both are Xenotar type formulations, with five elements in four groups (and probably the fastest Xenotar types ever made), but while they share that optical formula, the element parameters are not the same.
. The M42 1.6 is a modest lens in terms of build, using a lot of plastic, but has rather unique bokeh. This seems to be a five element lens in four groups.