On PM packaging and in many fan dioramas bayonets are shown affixed to muskets like this:
However this is not correct. Bayonets in real life were always affixed at the tip of the musket, with the blade usually on the right side, like this:
Proof
here,
here,
here,
here,
here, and
here.
Fixing the bayonet in this way is not only historically accurate, it also looks a lot better.
There's a new style musket about. It has a front sight, which prevents the bayonet being fixed to the side, so it must be mounted underneath, like so:
The old style musket can have its bayonet mounted like this too:
(I suspect the front sight was not universal on muskets, and perhaps was used only on rifles of the period, but I am not certain.)