This part is listed by BMW as BMW part number 18101405737 (18 10 1 405 737) and is described as Genuine BMW 60MM ID Exhaust Gasket Ring for E46 M3.
Note: E46 M3 uses 2 different size seal rings for this connection. This is the larger 60mm ID ring. Please see below for BimmerWorld recommended related items.
BMW Fitments:
3 Series
E46 (99-05) > M3