Window envelope (destination unknown) from MATI Mulhouse (Manufacture Alsacienne de Tissus Imprimes = Manufacture of Printed Alsatian Fabrics).
Postage is 25c sower and 2 x 1fr Peace definitives postmarked MULHOUSE R DE FRANCE / HAUT RHIN 4-4 39.
Message on reverse in French translates as:
If you are sellers do not miss to exhibit at the fair of Mulhouse. If you are buyers do not miss this important event which visit aua place from 28 May to 11 Jun 1939. Information: City of Mulhouse