The Bilstein e36 M3 rears have replaced Bilstein xi rear shocks.
There is just one rear xi Bilstein shock now. They can be paired with either the Sport or HD. It's the exact same part number as the e36 M3. Other RWD e46 also got an updated 46mm rear shock, but it's a different part number.
The bilstein rep said they tracked all of the failures and that they occurred almost exclusively in the northeast from Virginia up though Canada.