Apparent magnitude in the z band as a function of the observation date. We compare our new LBT/LBC measurement in 2021, with magnitudes derived from the CFHT Legacy Survey, PanSTARRS PS1, and MzLS datasets (2003, 2011, and 2016, respectively). The solid vertical line marks the time of the 2018 Chandra observation, which detected bright X-ray emission from J1641+3755, while the dashed vertical line marks the time of the 2021 XMM-Newton observation that did not detect the QSO.