O2 Sensor ?
 

This may be a dumb question, but I'm relatively new to 911s. I decided to remove my heat exchangers on my '75 911 S and replace them since they were rusting. I also pulled out the cat, and plan on replacing it with a cat bypass (since historic vehicles as exempt from emissions testing here). Anyway, I was looking for the O2 sensor and couldn't find it....anywhere. I am wondering if the previous owner unplugged it and adjusted the mixture to run lean. It doesn't seem I've been running very rich, but I'm a little concerned what this did to my engine. Where does the wiring run so I can put an O2 sensor in, and will I need to readjust the mix if he's tinkered with it? Thanks

1975 911's did not use an O2 sensor.

Oh, well that makes it easy! Thanks :)