Family #125 in the 1798 Grimm census.
Family #10 in the 1834 Grimm census.
Maria Katharina Fries was born about 1765, possibly in the Colony of Dobrinka, before her family settled in Grimm. Her family were Germans who immigrated to Denmark in 1762 and departed after May 1764, with their destination Russia. Early settlers who arrived before the Volga villages were prepared and ready for their occupation typically settled in one of the few villages available before that year, such as Dobrinka. These families spread out to other colonies once they were available.
It follows Maria Katharina Fries was from the only Fries family in Grimm: Katharina Elisabeth, a widow who re-married to Johann Georg Schmidt. She would have been about 10 years old in 1775, so her name should be in the 1775 Grimm census.
1775 Grimm Census 
Her mother Katharina Elisabeth is listed with two daughters from her first husband, Johann Georg Fries:
Neither of these names is a match with Maria Katharina. Believing that she did not survive would be easy, except she appears in the 1798 Grimm census as the wife of Johann Georg Konrad Grosch. It's possible that the census taker wrote down the wrong name; either of the names above has one of her two given names. I checked my records for the names of her sisters, and neither show up in any further Grimm censuses. This is not to say that they never existed, because they could have married and moved away to another village before the 1798 census. But it's just as true that one of those women should have been Maria Katharina. The situation is one to consider if you are studying this family line.
1798 Grimm Census 
Maria Katharina is not actually mentioned in the 1834 census, but her husband and children are. 
According to a note in family #10 of the 1834 census, Mattias Schwan was her husband Johann Georg Konrad Grosch's step-son. Initially, I interpreted this to mean that he was the son of Maria Katharina Fries of this profile. Closer examination, however, shows Mattias Schwan had a different mother.
Georg Konrad's children by Maria Katharina Fries were:
His step-son Mattias Schwan was born in 1802. The census does not state he is illegitimate, but definitively that he was Georg Konrad's step-son.
There is a gap between the birth of Anna Katharina, born in 1803, and Johann Tobias, born in 1810. I believe that's because Maria Katharina, Georg Konrad's first wife, passed away between 1802-1808.
After her death, Georg Konrad remarried an unknown woman whose name was never captured on a census. That woman was a widow herself, after losing the father of her son Mattias Schwan, born in 1802.
At some point between 1803 and 1809, this unknown woman married Georg Konrad, and in 1810, the couple had their only child, Johann Tobias.
Maria Katharina's husband's second wife passed away before 1834, making her one of a group of Grimmers whose names were not captured by a census. Unless her descendants have a bonanza of old family records, we will probably never know her name.
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