The replacement property for a modal logic states that:
$\text{ If } (p \leftrightarrow q) \text{ ,then } (\Box p \leftrightarrow \Box q)$ sometimes modal logics closed under this property are known as congruential. I am trying to find the first reference to logics that do not validate this property. I am looking for the first reference to a system that one could call the weakest modal logic i.e. it is simply a propositional logic in an extended langauge by a modal operator $\Box$ which is meaningless (does not have any specific modal rules nor axioms). The earliest reference I found is "An Essay in Classical Modal Logic" by Karl Krister Segerberg from 1971. I would appreciate any help with finding references that are older than the one provided.