CH 1943 W14 Bloc "Zurich 4+6" FDC with address (registered mail)

Switzerland 1943 W (Semi-postals and Special issues sold at an additional premium)
anniversary bloc "100 Jahre Schweizer Postmarken Zürich (Zürich 4+6)",
FDC white cover address (letter of Marken-Müller), with additional Pro Juventute stamp for valid tax of the registered mail to Grabs,
cancelled with special red first day cancellation "26.II.43 Zürich - 100. Jahre Schweizer Postmarken",
on the back side arrival cancellation "27.II.43 GRABS (St. Gallen)", luxury quality, 
Catalog SBK W14 and J104, Michel Bl. 8 and 415.