Well, it turns out this family heirloom is a SxS 1878 Colt and I won't be planning any conversions at all. Spent some time in the shop getting SOME of the century old grime off of it and have to say I'm impressed...this is a high quality shotgun.
I pulled both locks and they fit so tight that the wood in the lock mortises looks new, yet the locks were easy to remove...only one small lock screw...the front edge of the lock plate has a lip on it which slides in/under the frame then the whole lock panel sits down and the one screw holds it in place.
They have v-springs...for the world they look like medium size versions of typical Flint lock assemblies I'm used to...tumblers have no half cock notches/full cock only...internals are clean as a whistle and in immaculate condition...look virtually new.
I'm beginning to think most of the initial aged look on this old smoothbore really is from the external grime and banging around abuse from a century of ownership, moving in and out of different houses, in and out of closets, attics, etc.....as opposed to actual use, wear & tear.
This may not have been fired all that much...the bores are a little rough but more of a surface scale than pitting...might have suffered from corrosive primers and simply never got REALLY 100% cleaned like they should have been. A wire brush with steel wool around it chucked in a drill motor got a lot of it smoothed out...need to finish getting them smoothed the rest of the way.
The action and lockup is very tight... .12ga shells drop in and come right out normally, the chambers have no step, just a normal looking taper so I wouldn't think any odd shell length is called for...the shells rise up on extractors but there are no spring ejectors.
I went up and down each side of the top and bottom seams between the damascus barrels with a brass utility brush & WD40, got all the grime and crud out of there and the bare metal looks perfect...not a spec of rust or corrosion.
This Colt double is actually quite a quality built smoothbore and other than the fact that it was temporarily dirty, and the bores could be polished, its in very solid shape...a shooter for sure. I have 1000's of once fired .12ga AA and STS hulls...plenty of 209 primers and Goex...ordered some Circle Fly .12ga wads & cards today...will try some trap targets, hunt it a few times...should be a good experience