I think it is from 1850's ?
If you are right, then it's probably an insurgent flag.
One detail that I have never seen before is that the eagle is facing right (looking at his left wing).
That is strange
What makes you think it comes from 1850s?
Certain material hold themselves only for a certain time. If it was older and not taken care of by "professionals" it would look a lot worse. As this person haven't told us if his family are experts at keeping flags in good quality I can only assume that it must be very recent (just as he did)
and any Polish flag after 1918 would look more like the official flags of Poland. Anythign else before 1918 may be very different as the symbol of Poland wasn't defined to a very specific white eagle. People generally agreed that their symbol is an white eagle. But how it looked?
Also I guess this must be from the province of western Poland as it has no Lithuanian or Ukrainian symbols on it. Only the eagle.
It may happen that it's location may up close to Poznan as the eagle on the herald of Poznan faces right-wards.