The two cars are not the same configuration. Both cars sport a rectangluar hole on the outer part of the sidepods (near the tyre) this was introduced at least as early as Monza. The rear view of the car has the new periscope exhausts visible between the other holes and the middle of the car.

The outer rectangluar holes are clearly not for cooling, they are cut into a part of the upper sidepod that is purely a (single skin) extension of the sidepods\flip up forming a deck for aero purposes, if you drilled through this section it would go straight through to the other side, moving inbaord a litle towards the exhausts it would drill into the engine compartment. so it cannot be or cooling as there is no radiator outlet under or ahead of the hole.

General shape

The next guess is it would be for aero reasons. The first clue is looking at where the hole starts, it sits inline with the flip up, knowing the sidepod is undercut at this point and the downward sweep of the engine cover, my guess is that it is help the flow off the underside of the flip up. Looking at how the flip up works, there's a clean flow (yellow arrow) over the flip up, but its the underside that does the work. the underside directs the most of the airflow up over the tyre (red arrows) with no sideplates underneath some of the flow spills off the flip up and under this section of deck (red striped arrow), this must upset the flow and perhaps slows down the flow in the coke bottle area.

without the hole



Putting the hole there allows some off this flow to direct up over the deck (red striped arrow) improving the general flow in the area.

with the hole

This Hole added to the new sidepod shape and the fins leading the sidepod inlets suggest this the flow along the sidepods and over the floor is an area causing Williams problems, as they constantly are seeking improvements