Hello Josef. I too have in my collection one of this type covers from the cancelled 1936 "21 SAF"
. The cover is a duplicate to the one you have shown accept of course with different R. No (435). The information i have about it is that it was intended for the 1936 "21 SAF" which was scheduled to be operated by the LZ 129 Hindenburg departing from Frankfurt on the " 2.December.1936
", but the flight was cancelled so all the mail was sent on the "3.December.1936
" by the "DHL" German Catapult flight, Catapult service to "SOUTH AMERICA" arriving in "RIO" on the 5.December.1936. On the back of your Cover it states the Lufthansa flight was on the "3.December.1936
As for the Red confirmation stamp with the letter "d", in the "MICHEL" Zeppelin catalogue on the 1936 "1 NAF" " Liechtenstein Mail - quotes " The stamp used in Frankfurt/M with the distinctive letters "c" and "d" was used as a confirmation stamp (BSt). So maybe the mail was cancelled at Frankfurt/M with the Red BSt (d) and then loaded on to the Catapult flight