I always have called them steel inserts, too, but in a recent article in the Star Magazine, Stu Ritter called them "copper inserts".
It doesn't really matter anyway. We can all be assured that Mercedes redesigned the gaskets to have some kind of metal inserts that prevent them from leaking.
2001 E430, Bourdeaux Red, Oyster interior.
1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".