Fortunately the brass ring on my vehicle does stay in place with a tight fit upon test fitting it, so I will use grease on the o-ring and secure it in place before attempting other solutions.
I think Dave is correct that the previous owner must have tried to reinstall the ring while the mixture unit was still on the intake, and used gasket glue to solve the problem - but that just created a mess, which might also explain some of my vehicle's hunting issues from before.