Family #60 in the 1767 Grimm census.
Family #136 in the 1775 Grimm census.
Family #5 in the 1798 Grimm census.
Peter Walter was born about 1739 in the Darmstadt area of what is now Gemany. He married Anna Margaretha Schnergard by 1766, and the couple had their first child in June of 1767.
Peter and Anna Margaretha decided to immigrate to Russia with other German immigrants in 1766. They departed Luebeck on the ship Apollo and arrived in St. Petersburg on 29 August 1766. 
The couple spent the winter in Oranienbaum before departing for the Volga River region of Russia the next spring. They finally settled in the village of Grimm. Their first child was born shortly before they arrived in the village on 04 July 1767. They are listed among the first settlers of Grimm.
1767 Grimm Census 
By 1775, the census shows her husband Peter married to a different woman, Anna Maria, who was the same age as he was. It does not, however, show that eldest child Johann Jakob was the son of Anna Margaretha, his first wife, as was typically done when a widowed spouse remarried. It could have been that the census taker misidentified Anna Margaretha as Anna Maria, but then one would have to believe that the census taker also wrote down the wrong age for her or that there was a typo in the record. The Anna Margaretha of the first census was 10 years younger than her husband, but typing 18 instead of 28 would be an easy error to make. By 1798, still not identifying a change of spouse, Peter's wife is identified as Anna Margaretha Schnergard, the same name in the Kulberg Reports and 1767 census, and she is the same age as Peter, not 10 years younger. It would be easy enough to believe Peter had three wives, except that not one of the last two is designated as a second or third spouse and none of his children are identified as belonging to a different mother.
Based on this information, I am treating Anna Margaretha and Anna Maria as the same person. There must have been an error with her age the 1767 census, however it was made, and instead of being 18, she was 28 years old. In the 1775 census, the age discrepancy was corrected, but the census taker inadvertently wrong down the wrong first name, Maria instead of Margaretha. By the time of the 1798 census, both her correct name and age were used. All the children had the same mother.
1775 Grimm Census 
By 1798, daughter Anna Barbara was probably married and living in her husband's household. The couple had at least one more child, Johann Just, born in 1778.
1798 Grimm Census 
Peter is not mentioned in the 1834 census, although his sons and grandchildren are.  He most likely passed away before the 1816 census, where his death would have been noted. Had he survived to 1834, he would have been 95 years old. If he survived to 1815, he would have been 76, which would have represented a full life for a Volga German during that time frame.
It is an established fact that these families intermarried with each other throughout the generations in Russia. Most probably knew each other from their time in Germany.
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