  The solar activity during the Maunder Minimum (MM; 1645 -- 1715) has been
considered significantly different from the one captured in modern
observations, in terms of sunspot group number and sunspot positions, whereas
its actual amplitudes and distributions is still under active discussions. In
its core period (1650/1660 -- 1700), Martin Fogelius and Henrich Siverus have
formed significant long-term series in the existing databases with numerous
spotless days, as the 13th and 7th most active observers before the end of the
MM. In this study, we have analysed their original archival records, revised
their data, have removed significant contaminations of the apparent "spotless
days" in the existing databases, and cast caveats on the potential
underestimation of the solar-cycle amplitude in the core MM. Still, they
reported at best one sunspot group throughout their observational period and
confirm the significant suppressed the solar cycles during the MM, which is
also supported from the contemporary observations of Hook and Willoughby. Based
on the revised data, we have also derived positions of notable sunspot groups,
which Siverus recorded in 1671 (~ N7.5{\deg} +/- 2.5{\deg}), in comparison with
those of Cassini's drawings (~ N10{\deg} +/- 1{\deg}). Their coincidence in
position and chronology in corrected dates indicates these sunspot groups were
probably the same recurrent active region (AR) and its significantly long
lifespan (>~ 35 days) even during the MM.
