As you have noticed in the Introduction page of the stamps of the United Nations, not all the stamps have been created or engraved by the same people. Jean Van Noten who has done the stamps 0.50F, 1F, 2.50F, 20F of the post serie, and the 5F, 6F, 8F stamps of the airmail serie has always created proof stamps of his own creation. They are quite hard to be found.  

First stamp value 1F with original color purple and proof in dark grey color: 

Stamp value 2.50F with original color: 

Stamp value 20F with original color red and proof in dark grey color: 

Stamp value 8F airmail stamp with original color dark brown and proof in light brown color: 

Stamp value 1F stamp with color of 7.50F airmail: 

A complete other proof is printed in black and white with some slight differences with the original stamp. Printed on hard glaced paper, exact same format as the real stamp. The same type of proofs is issued for US stamps. Additional information is always welcome.