Similar to most other teams, Williams have introduced a new, one-off rear wing to adapt to the requirements of the Monza Autodrome. Focusing on top speed, drag shedding is the easiest with a smaller rear wing, given that the rear wing generates much more drag per point of downforce compared top the diffuser. This also explains why teams are never modifying their diffuser, simply because it's more efficient and probably more complicated to alter as well.
The new rear wing fitted in the FW36 features an identical endplate as the one in Belgium, except for the reduced amount of louvres. This has been reduced to one as more are hardly needed due to the smaller angle of attack of the rear wing. The design of the rear wing itself means there are less strong wingtip vortices anyway, so less louvres are needed to help reduce them.
Perhaps the most interesting design feature of the team's Monza rear wing is that they put in the effort to create a new fairing for the DRS activator in the middle of the wing. While the one at Spa was still a big hub to help control airflow under yaw, the new one is clearly aimed to create as little of an obstacle to the airflow as possible. The support is as narrow as possible, and the thicker, bullet-like fairing follows the direction of airflow as is drops down over the bend of the rear wing's main plane before being kicked up.