Here is a very early FDC , unofficial ( RO Post Office in '30s did not issue FDC ) with 1936 " Kleinen Entente" 2 stamps , (Michel nr.522-523) and having as the addressee " Wilhelm Nathansohn " a well known stamp dealer from Bucharest.
It is a registered letter and in addition to 2 " Kleinen Entente " stamps , there were added another 2 stamps " Trimiteri Postale - Fondul Aviatiei " 1936 (
Postal sending - Aviation Fund ).
All the stamps were cancelled with the
postmark dated 1 December 1936, which actually is a FDI cancel because the "1984 Romanian Catalogue" gives 1 December 1936 as the date of issuing these stamps with overprint : " Tchekoslovakia - Jugoslavia- 1920-1936 " .
Also same catalogue gives year 1936 ( no specification for day & month) for the other 2 stamps " Aviation Fund "issuing.
But surprisingly Michel Catalogue ( I have one from 2008) gives the date of 5 December 1936 as the FDI !!!
According to 1984 RO Catalogue , 54,800 sets were over-printed to celebrate " Kleinen Entente ", sets which were in circulation until 1 September 1937 when Post Office withdrew them.
The back of the above cover . It shows that Willy Nathansohn , the stamp dealer, wanted all his items to be by ... the book ! :)