Netherlands 1930 (May 1), KLM First flight Amsterdam - Sofia, Express airmail hand stamp in purple, cover bearing mixed franking of 1924/1925 2½ c  Flying Dove  & 1925 Queen Wilhelmina 5c. Arrival May 1st 1930 postmark on back. Also marked on back by hand '99'.  Enclosed with Original Filing slip written with various details mentioned.