Nice picture, M.Scottishmaiden !
About the exact location of the post office : it was the one on place du Dôme (maybe there isn't anymore there). The killers strangled the night watchman and stole 230.000 francs on December 28th, 1920. They were condemned twice, once in Strasbourg on February 24th, 1921 (less than two months after the crime) but the verdict was "revoked" because of an error in the trial ("vice de forme). So they were condemned for the second time by the Cour d'Assises du Haut-Rhin, in Colmar, the next 12th of July.
On October 20th, they were both beheaded in front of the old prison of Strasbourg, rue du Fil. Indeed, young André Obrecht (he was 22) was there but only as a eyewitness. So was he for Landru's execution, four months later. He became second assistant on April 04th, 1922, and did his very first execution in Paris on May 23rd.
By the way, ny name is Sylvain.
_________________
Je ne suis pas un poisson. Je ne suis pas un capitaine. Nemo n'est personne. Mais il compte le devenir.