As others have said the abbreviated date stands for the 21st of the first month of Adar.
It would be very hard to determine if one of the people on this list is Henriette with only the limited information we have. There is a Yetchi daughter of Yitzchak (#676), this could be her if we assume that Itzig was her father's name and was short for Yitzchak, but there's no real reason to conclude this without more information.
Do you know her husband's name because some of the women are listed as wife of ...? Also, as this list seems to be part of a larger record, is there a reason that you believe Henriette is on this page specifically?