Went lean...

If it were my car, I'd get rid of that breather thingy, or fix it so it can't rotate out of position again and block the choke. Better yet, just run a filtered breather rather than having that thing hooked to the air cleaner. I'd rather wipe off valve covers occasionally than burn oil. Besides, if something goes wrong (overfill of oil, broken ring, bad head gasket), I'd rather the breather make it obvious by rust-proofing my engine bay than by burning a ton of oil that's sure to cause additional repair costs. Just my $.02

I'll also add that such a significant change in AFR should be felt by almost anyone, and that changes in performance should be cause enough to look for what's different, or got bumped the last time something was tinkered with.
The WBO2 helped in this situation, but I'd hope a shift from 10:1 to 20:1 would cause enough difference for even a causal driver to think 'wtf just happened' ;)
Just as many people who find a problem that's indicated by an O2 sensor as chase a bad sensor with tuning adjustments - and the internet being what it is, folks will chase all kinds of odd things based on a vague description. Luckily in your case, you had corroborating information that made trusting the gauge a wise choice. That's not always the case, and luckily people reading this thread years down the road will know that based on some of the questions and clarifications posted.