Family #37 in the 1767 Grimm census.
Anna Maria was most likely married to an unnamed gentleman by 1763, and they had a child, Johann Friedrich, born in 1764. Her husband did not survive the cold winter in the St. Petersburg area or the treacherous journey to the Volga region of Russia. By August of 1767, she had remarried to Kaspar Hepp. Kaspar may have formally adopted Johann Friedrich, or he may have simply told the census taker that the child had the same surname as he did in order to simplify the record keeping. In the end, there is no other surname identified with his name beyond Hepp.
1767 Grimm Census 
Neither Anna Maria nor her son Johann Friedrich are listed in the 1775 census with Kaspar Hepp. Most likely, the two were casualties of the early years in the Volga colonies. Both were deceased before the 1775 census was taken, giving her widowed husband the opportunity to remarry.
1775 Grimm Census 
There is a problem with his age in this census; based on his age in 1767, he should be 8 years older in 1775, or 37. The age "27" is most likely a typographical error. Also note that he has a different wife than the one listed in the 1767 census, and that his son Johann Friedrich is not included in this census entry.
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