It is very interesting. There is much written about where and when the unisono or octave stringing of the 4 and 5 courses. The only thing mentioned about the first course is that it was mostly single. I could never find a written explanation for that fact in the historical literature. Maybe someone else did find more details in the literature.
Gert Petersen 2000, Spr-BRW, Felipe Conde FP17 Negro, Santos Hernandez 1932 S-BRW, Asturias 10 string s-IRW, Ramirez 1a Especial 2014, Bernabe M5 S, Bert Kwakkel Merula 1982, Baroque guitar by Martin de Witte