Birch's paper
“Noncongruence subgroups, covers, and drawings”
(pages 25–46 of *The Grothendieck Theory of Dessins d'Enfants*
(Leila Schneps, ed.), LMS Lecture Notes Series #200) appeared in 1994,
though John Voight notes a hint of it as early as a 1971 paper
“Modular forms on noncongruence subgroups”
by Atkin and Swinnerton-Dyer, pages 1–25 in
*Combinatorics*
(*Proc. Sympos. Pure Math.*) **19**, bottom of p.6:
“Also the number of unknowns *c* can be cut in half
at once by observing that
*dj* / *dζ*
has factors
*F*_{3}^{2} · *F*_{2}.”