Monaco-Heliport and Monaco-Port have immigration/customs officials -- at least at some times. IIRC those authorities are French and/or at least not subordinate to Monaco but are subordinate to France; in other words, it would be France that carries out the external border controls at those two authorized external border crossings into the Schengen Zone, and that is not a result of a Monaco-Schengen Agreement but that is a result (at least initially) of bilateral agreement(s) between France and Monaco.

I've not received any stamps at Monaco's heliport or port, but some have. I wouldn't be shocked if it is a French stamp.