Nano, the one cent Franklin plate number block of six stamps that you picture on cover is Scott No. 632, a rotary press issue that is perforated 11x10½. The quick and definite giveaway is the plate number 21621. When a plate number is involved the Durland Standard Plate Number Catalog can reveal quick identification of a stamp.
Nice air mail cover with a slightly out-of-the-norm franking.
No need to purchase catalogs, plate block info is freely available online. Plus, it has a 'reverse lookup' which support entering the plate number and instantly having it return any stamp which used that plate number. http://stampsmarter.com/features/Pl...ockHome.html Don