My "metal filter cap" term refers to the thing you grab with your oil filter wrench - the whole big thing on the left in your photo. The big O ring goes on it,
but I understand that you can unscrew the thin stalk (that you slide the filter on) from this external fitting. I suppose they can be damaged and may need
to be replaced. Though rarely removed, an O ring is provided in case you do. Haven't been there - haven't done it - just remember reading this somewhere
in the ethos. (Strange that I can come up with the word "ethos" yet struggle to identify
the parts as "thingys" and "whatchacallits". I think I'm missing an O ring in my head. )