Brussels local in 1919

This cover was sent from Brussels to the Hotel Metropole in Brussels on 19 May 1919. It has two Red Cross stamps from different issues.

Characteristics: 10c+10c of 1915 with 20c+20c of 1918 Exact postage of 30c Clear, sock-on-the-nose cancel on the 10c+10c Circular cancel "357" (mailman's cancel) No cancel of arrival (but, was it needed?)
Estimated value: $90 [Auctioned by Olivier Simons in August 2006 for 75 euros] [Auctioned earlier by David Delhaye in April 2006 for 34 euros]