Screw size holes wouldn't be for ventilation in that size surface,
IMO. With a house that old, there would likely be plenty of
"ventilation" elsewhere, especially around an old window.

There may have been some additional/temporary framework, added onto
the interior, to put some sort of shielding against a draft coming
through the window unit. Is there evidence of other screw holes along
the sides and top, maybe, to indicate something was attached there, as
well?

As someone said, there isn't enough info for us to speculate or reason
more so.