Heinz Loewenstein lived in Haarlem. He was a member of the 'Oranje Vrijbuiters', a team of heavies who were active in Utrecht. They eliminated collaborators, collected intelligence and stole food-stamps. In August, September and November 1943, most Oranje Vrijbuiters were arrested. Heinz Loewenstein was one of them.
In February 1944, 20 team members were put on trial, all were given the death sentence. 18 members were executed on 29 February 1944 on the Waalsdorpervlakte. The 25 year old Heinz Loewenstein was amongst them.
NIOD, Erelijst Verzet en Koopvaardij, database made by J.W. de Leeuw;
B. Braber, Zelfs als wij zullen verliezen. Joden in verzet en illegaliteit in Nederland 1940-1945 (Amsterdam, 1990) 130
A biography and a photo can be found on the website of the Erepeloton Waalsdorp.