This is one of the more unique covers from my Equatorial Guinea collection
The letter was sent by the Central Bank of Equatorial Guinea to the the International Monetary Fund in Washington DC on the 26th November 1975
What makes it so unique is that one of the stamps used is what I call a "wall paper" issue. It is one of those issues listed under the Appendix Section in Stanley Gibbons for stamps considered to have been "issued in excess of postal needs....." Front BackStamp
This is part of a set of 7 stamps illustrating the paintings of Spanish painter El Greco that were apparently issued in 1976 according to SG.
The noted issue date doesn't match with the "cancel" date on the cover itself Bank Stamp
The round stamp on the back reads as "Banco Popular de la Republica de Guinea Ecuatorial" (another name for the Central Bank of Equatorial Guinea)