More research needed...
A little more research on the presumed artist, Francis Boucrot.
1. Boucrot is a very rare family name. In terms of Google hits:
"Boucrot": 147,000 hits, many are for "Marc Boucrot", a contemporary film editor, or "Emanuelle Boucrot", an Associate Professor of Molecular Cell Biology.
"Boucrot, F.": 148 hits, most are about the artist active in the 1950s and 1960s, who illustrated children's books, and did marionette theatre.
"Boucrot, Francis": 582 hits, most are about the artist active in the 1950s and 1960s, who illustrated children's books, and did marionette theatre.
"Boucrot, Francois": 81 hits, none seem to be an artist, some are genealogy hits from people in the 1850s.
So there only seems to be one artist named Francis Boucrot from Google searches.
2. Francis Boucrot.
Born on October 16, 1920 in Nevers, Nièvre, Bourgogne-Franche-Comté (France)
Died on August 18, 1992 in Paris 12e Arrondissement, Paris, Île-de-France (France)
He was active in illustrating children's books, including a book on Shadow Puppetry "Les Theatres d'Ombres", and did posters for theatre and film productions.
3. Signatures (most recent first, sorry for the poor image quality).
a. From poster "Youm et les longues moustaches"(1961)
b. From book cover "Fabuleux Petrole"(1960)
c. From poster "200 livres pour nos enfants"(1958)
d. From book cover "Air Avions Fusees" (1958)
e. From book jacket of "Les Theatres d'ombres"(1956)
f. compare to signature on the stamp (1955?)
The signature from the 1956 book is perhaps the closest to the one on the stamp. Most telling is the small caps "f" for the artist's first name, with rounded curves.
Conclusion: Based on the rarity of the last name "Boucrot", the inability to identify a second artist by the name "F. Boucrot", the time period (mid 1950s) suggested for the production of the stamp (25th anniversary of the canonization of the 3 martyrs) and when this artist was most active, and the overall resemblance of the signature on the stamp to the artist Francis Boucrot's signatures on works attributed to him from this period (even though the signature seems to have evolved a bit over a short time period), the most parsimonious conclusion is that Francis Boucrot is the artist behind the Ondessonk stamp.