Dude, save yourself a lot of pain and go to the local MB store and buy them. They are stainless steel and if you get some cheaper ones at the local hardware store they will be the same price and will weld themselves to the bolts in the trunk insert.
I don't know what dealers do here in the UK but almost every Mercedes you see more than a couple of years old has nasty rusty run lines staining the rear license plate (ours are normally plexiglas)from horrrible mild steel cross head screws.So I cut,ground and mirror polished four stainless steel 6mm allen bolts.
They look brilliant.