  HD 166620 was recently identified as a Maunder Minimum candidate based on
nearly 50 years of Ca II H & K activity data from Mount Wilson and Keck-HIRES
(Baum et al. 2022). These data showed clear cyclic behavior on a 17-year
timescale during the Mount Wilson survey that became flat when picked up later
with Keck-HIRES planet-search observations. Unfortunately, the transition
between these two data sets -- and therefore the transition into the candidate
Maunder Minimum phase -- contained little to no data. Here we present
additional Mount Wilson data not present in Baum et al. (2022) along with
photometry over a nearly 30-year baseline that definitively trace the
transition from cyclic activity to a prolonged phase of flat activity. We
present this as conclusive evidence of the star entering a grand magnetic
minimum and therefore the first true Maunder Minimum analog. We further show
that neither the overall brightness nor the chromospheric activity level (as
measured by S$_{\mathrm{HK}}$) is significantly lower during the grand magnetic
minimum than its activity cycle minimum, implying that anomalously low mean or
instantaneous activity levels are not a good diagnostic or criterion for
identifying additional Maunder Minimum candidates. Intraseasonal variability in
S$_{\mathrm{HK}}$, however, is lower in the star's grand minimum; this may
prove a useful symptom of the phenomenon.

Solar and Stellar Astrophysics