Module Hash_set.M

M is meant to be used in combination with OCaml applicative functor types:

type string_hash_set = Hash_set.M(String).t

which stands for:

type string_hash_set = (String.t, int) Hash_set.t

The point is that Hash_set.M(String).t supports deriving, whereas the second syntax doesn't (because t_of_sexp doesn't know what comparison/hash function to use).