Looking at the photo of the underside of your barrels makes me think that perhaps they were refinished with DuraCoat or Cerakote firearms finish. It looks like there is some scroll engraving on the barrel lug that is completely filled in except where the finish has flaked off. I'd lean toward it being DuraCoat because it looks pretty thick, and Cerakote is typically only .0005" to .001" thick. Cerakote is also usually baked on, and I don't know what temperature is used for baking or if it would melt soft solder joints on double barrels.
Of the two, I'd hope it was the DuraCoat just because Cerakote is supposed to be much tougher to remove. Even DuraCoat is said to be pretty tough to remove. It is suggested to use Jasco Paint and Epoxy Remover, because most other chemical paint strippers won't remove it. Jasco is sold at Lowe's and Home Depot for about $10.00 a quart. Even with the Jasco, it is going to take some wire brushing. Jasco is a methylene chloride stripper, so you'd want to use rubber gloves and eye protection.
