Pouce arrière (m)
(with) definite article: lehole for the thumb of the upper hand
According to Blanc, "the 8th level of the scale"; NB: Because all thumbholes are (due to anatomical reasons) necessarily located at the back, the adjective is in principle superfluous; The only motive for its addition I can imagine, is the understandable desire (need) to distinguish the term from the common noun that is also used as a unit of measurement.
Blanc, Bernard: Cabrettes à l'étude. Description technique de la cabrette. In: Ricros (et al.): La cabrette… (2017), p.596.