Yep that large o-ring does not come in the package with the pre-filter.

The pre filter looks like something for a miniature coffee machine. If it is not too dirty you could conceivably blow it out with brake spray and put it back. But definitely change the o-ring if you open it up.

Be sure the bore where the o-ring sits is clean. Mine was not and it was causing a tiny air incursion and rough idle.

__________________
The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech