According to the Death Index (Sterbebücher) of Auschwitz, Isak Elkan van Polen died there on 5 September 1942. This date differs rather much from the official date of death of 30 September 1942 as established after the war by the Dutch Ministery of Justice. For the time being, the website joodsmonument.nl only lists the dates of death, as published at the time in the Dutch State Gazette by the Ministery of Justice.
The death of Isak Elkan van Polen according tot he death index of Auschwitz (Sterbebuch).
(click the link and wait for opening of the deed in a new window).