Illustrated Envelope of the China Clipper Airplane
This nicely illustrated cover is not really a commemorative for the China Clipper, but is actually a First Day Cover for the Hawaii stamp of 1937. The purple vertical stamp to the left of the clipper airmail stamp is the new issue. The cancellation is the official first day cancellation used in Honolulu.
This is an "unaddressed" first day cover. These were created as a courtesy to collectors and publishers of the covers. They were handled in bulk to lessen damage in the mails.