Consider this stanza from Byron's Don Juan:
What then? I do not know, no more do you.
And so good night. Return we to our story:
'T was in November, when fine days are few,
And the far mountains wax a little hoary,
And clap a white cape on their mantles blue;
And the sea dashes round the promontory,
And the loud breaker boils against the rock,
And sober suns must set at five o'clock.
The setting suns would suggest the author meant 5 p.m., and not 5 a.m.
To the best of my knowledge, with a few notable exceptions, abbreviations in general were not part of everyday speech before the Twentieth Century. (What forced them into everyday speech is a whole different story, I suppose).
I've done some research but, to my surprise, I couldn't find anything that would even remotely suggest when, where, and why the abbreviations for "post meridian" and "ante meridian" became standard everyday terms, replacing all those clunky yet, in my humble view, somewhat romantic, if not downright poetic, "half past eight in the evening," "two o'clock in the afternoon," etc, etc.