The author intentionally broke with the tradition of labeling moves (and points) with letters “A”-“T” (excluding “i”) and numbers 1-19. Two lower-case letters in the range “a”-“s” were used instead, for reasons of simplicity and compactness.
Also a forums discussion: OGS 19x19 boards marked A-T? Why is " i " missing? - it’s funny, the poster of that topic was also programming a Go application!