M is meant to be used in combination with OCaml applicative functor types:
type string_to_int_table = int Hashtbl.M(String).t
which stands for:
type string_to_int_table = (String.t, int) Hashtbl.t
The point is that
int Hashtbl.M(String).t supports deriving, whereas the second syntax doesn't (because
t_of_sexp doesn't know what comparison/hash function to use).