CH 1944 F40 Pro Aero Special Flight ill. FDC Genève-Zurich anniversary mail flight with vignette

Switzerland 1944 (Air Post stamps - Pro Aero) 
Pro Aero "25 Years Swiss Airmail" anniversary mail flight Genève-Zurich of 20.IX.44, SBK F40, Michel 438,
registered mail (subscrption) cancelled "20.IV.44 GENEVE 1 - Poste aérienne",
front side arrival cancellation of flight "20.IV.44 ZÜRICH 1 - Briefversand",
backside arrival cancellation "21.IV.44 BERN - Telegraph Eilzustellung",
with yellow flight vignette "Jubiläums-Postflug Genève-Zürich",
illustrierted cover with address, very decorative letter addressed to prominent person, luxury conservation.