The position of the pin arouses my curiosity as the renovation of "5 blocs" is a long time story and never heard about this neighbourhood there.

Probable correct position is near Rue Vandenbranden. 

See : 

https://www.bruxelles.be/sites/default/files/bxl/CdQ%20JaF%20-%20carte%20op%C3%A9rations%20mise%20%C3%A0%20jour%202015.pdf (pin 14)

https://monument.heritage.brussels/fr/Bruxelles_Pentagone/Rue_Vandenbranden/10003162

https://www.reflexcity.net/bruxelles/communes/ville-de-bruxelles/pentagone/rue-vandenbranden

https://collections.heritage.brussels/medias/objects/30/AVB_CI_C_11313.jpg is the more precise postion i found : "Le Coin du Diable et la chapelle Saint-Roch, vers 1940 © Photo Staelenberg. Archives de la Ville de Bruxelles, Collection iconographique C-11313"