Due to the lack of official documents on the one hand and demonstrable certainty, the Belgian government usually set a date of death of victims between the time of deportation and the end of the Second World War. In the official Belgian Government Gazette, the dates of death were then listed as the transport date of the deportation. The place of death was not mentioned too, as the exact place was not known.
In such cases, the Jewish Monument mentions "Unknown" as the place of death, and as the date of death the officially used Belgian date of death, which is usually the date of transport and deportation to the extermination camps.
Even though the Jewish Monument mentioned otherwise, you may assume, with a very high degree of probability, that Grietje Stokvisch-Boas and her daughters Rachel Sara, Sara, Eva and Lena Stokvisch, arrived in AUSCHWITZ on September 10, 1942 from Mechelen and were murdered that same day in the gas chambers of Auschwitz Birkenau.
Source: The Memorial of the Deportation of the Belgian Jews, description Convoy VIII (8), page 24.