Information: Wikipedia

A Stolperstein literally "stumbling stone", metaphorically a "stumbling block" is a sett-size, 10 by 10 centimetres (3.9 in × 3.9 in) concrete cube bearing a brass plate inscribed with the name and life dates of victims of Nazi extermination or persecution.

The Stolpersteine project, initiated by the German artist Gunter Demnig in 1992, aims to commemorate individuals at exactly the last place of residency—or, sometimes, work—which was freely chosen by the person before he or she fell victim to Nazi terror, euthanasia, eugenics, was deported to a concentration or extermination camp, or escaped persecution by emigration or suicide. As of 29 March 2018, over 67,000 Stolpersteine have been laid in 22 countries, making the Stolpersteine project the world's largest decentralized memorial.

The majority of Stolpersteine commemorate Jewish victims of the Holocaust. Others have been placed for Sinti and Romani people (then also called "gypsies"), homosexuals, the physically or mentally disabled, Jehovah's Witnesses, black people, members of the Communist Party, the Social Democratic Party, and the anti-Nazi Resistance, the Christian opposition (both Protestants and Catholics), and Freemasons, along with International Brigade soldiers in the Spanish Civil War, military deserters, conscientious objectors, escape helpers, capitulators, "habitual criminals", looters, and others charged with treason, military disobedience, or undermining the Nazi military, as well as Allied soldiers.

Ludwig Ruppel was born in Gotha, 14 December 1876 and died in Sobibor, 16 July 1943.

He reached the age of 66 years.

Hedwich Sophie Ruppel-Seckendorff was born in Nürnberg, 13 September 1884 and died in Sobibor, 16 July 1943. 

She reached the age of 58 years.

Ludwig Ruppel, his wife Hedwich Sophie Ruppel - Seckendorff and their son Heinz Herbert come from the town of Konstanz on Lake Constance, where Ludwig had a flourishing practice as a psychiatrist. The three register with the municipality of Domburg. Ludwig came to Domburg because he knew the village from a holiday in 1909 with his father Prof. Ruppel at the Strandhotel.

Hedwich does sewing and volunteers at a first aid post, Ludwig rents out rooms to tourists to earn a living. He opens Pension Dr. Ruppel and advertises in the Domburgsch Badnieuws.

There are no records for the three years from the outbreak of war until the couple's death. Father and mother Ruppel were murdered by the Germans in the concentration camp Sobibor on July 16, 1943. He was 66 years old, she was 58.
Son Heinz Herbert survived the war and got the Dutch nationality in 1952. He passed away in 1996.