CH 1943 W18 Bloc GEPH subscription FDC with address

Switzerland 1943 W (Semi-postals and Special issues sold at an additional premium)
memorial bloc GEPH Geneva (Genf) - National Stamp Exposition, "100 Jahre Schweizerische Postmarken (Doppel-Genf) 1843-1943",
FDC (subscription letter), PTT-cover with address sog. Gaudard-letter, cancelled with red special cancellation "17.IX.43 GENEVE - GEPH Exposition Philatélique Nationale", Top quality,
Catalog SBK W18, Michel Bl. 10.