Please see scan(s); what you see is what you get!

An EXPORTDRVO envelope addressed to "Messrs. Hahn & Co.Ltd., Admonton Wharf Tow Path, Angel Rd., London, N.18, England"

"EXPORTDRVO Zagreb, Jugoslavija" is pre-printed on reverse of envelope

Exportdrvo is a wood industry company in Croatia

The postage stamp used = SG 899 20d. red "Jablancia Dam" issued in 1958

CDS = "ZAGREB 1   5XII6120  R  ??????" where ? is Zagreb in a another language? 

"TISKANICA" has been stamped in blue ink onto the envelope. Tiskancia translated is Printed Matter