Optical continuum luminosity versus radio luminosity at 1.4 GHz for X-shaped (crosses) and control sources (squares). A small sub-region (“Region 1” marked by a rectangle inside the plot) is identified to provide a tighter luminosity match between the two samples. In the further analysis, Region 1 is also subdivided into two equal bins in the radio luminosity to assess a possible luminosity dependence of the black hole mass estimates.