Poznan was known as Posen and part of the German province of Silesia fromI believe that part of the Poznan Province as a part of Greater Poland
(Wielkopolska) that was annexed by King Friedrich II of Prussia in 1772
during first partition of Poland has became known as Netzedistrict.
During the second partition of Poland in 1793 the remainder of the Province
of Poznan was annexed by King Friedrich William II of Prussia and it was
known as South Prussia.
Posen was briefly reunited with Grand Duchy of Warsaw (1807-1815) but
following Napoleon Bonaparte defeat Prussia got province Posen back
(1815-1919) and Province was partitioned into the administrative districts
(Riegierungsbezirke) Posen and Bromberg.
Thus, Posen could not be the part of Silesia.