2019 Glc coupe 350d was 2017 e350 c207 .. +65 ford kuga 2.0 cdi zetec awd..
Aftermarket alloys usually come with a 74 mm ISH centre bore hole , spigot rings are fitted to this bore hole to reduce the hole size ,so the wheels can fit any car . If they are missing, your then relying on the wheel bolts only to keep the wheels central and steady.
The spacers you need to fit between the hubs and the wheel centre bore if you fit after market wheels or wheels with a bigger hole in the middle. Normally plastic or alloy. Makes sure the wheel is truly central. You will get movement without them as it's the hubs job to support the weight of the car...not the bolts....their only job should be the clamp the wheel to the hub tightly. Spigot rings can also stop the bolts working lose.
The rings are made high quality plastic. Original wheels do not require spigot rings as will be direct bored. (Picture is for illustrative purposes only, colour of ring may vary, rings cannot be seen once in wheel).
OK , so self inflicted wound by the OP . "Mmmmm I wonder what these two circular rings are lying on the floor the day after I replaced my front discs ?...I'll just put them over there for now ...."