The O2 I have looks like a fresh install. But I am going to replace it soon anyway due to the car running rich.

As for the second sensor after cats. As far as I know its there to make sure the cats are doing their job. Thats all. Many times in the forums the upstream is recommended to be replaced at regular intervals but the downstream only when a code is thrown. I even see this when I write programs for cars. Front O2s have variables, rears never do. Its also why the upstreams are heated and the rears usually not - no need.

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1991 560 SEL / 185k miles
1992 750il / 17k miles - project car