FOM: non-standard models

[Neil Tennant]

I would like to know the earliest documented observation that there are
non-standard models for the theory of arithmetic. I am interested not so
much in date of first publication, as date of first realization of this fact.
Can anyone on the list help me?
To reduce load on the list, private replies can be sent to tennant.9 at osu.edu
Thanks!
Neil e/Tennant