So you have a chamber sleeved gun that you might decide to chamber sleeve back to 16. Why bother? The pressures of a 7/8 or 1 ounce 20 are not that much different than a 1 or 1 1/8 ounce 16. Chamber sleeving from 16 to 20 is far beyond most gunsmiths level of abilities. Poorly done they would take too much metal out of the old chamber to make fitting the sleeves easier. That leaves a gap which is soldered or welded, leaving a easily seen seam. It sounds like your was done in an expert fashion. There are half a dozen smiths in the US who could do that job. If you called them all the one who did it might even recall the job. Not a common job after all.

If the chambers were reamed out for the sleeves, then you will need to have it reamed out larger to fit a 16 gauge sleeve. Why remove more metal in the chamber area? Shoot it as is or send it down the road with what you now know. I’d love to have a overbored 20. To me that’s what that gun is. Proof means nothing in the US other than a vague endorsement of original configuration.