What in hail is going on with the left barrel (the patch?)?
Just a guess,,but I'd think the reason for (someone) installing the rifled liner came about because of damage to the left tube in that area where the 'patch' is.
The damaged area was hammered back into position over the liner, draw filed and polished out.
When the liner was soft soldered into place, the solder would also fuse the damage & cracks of the tube also (not that it's needed for tube strength).
The rectangular decorative gold(?) plate was added to cover the split/blowout damage area including the soft solder showing through.
Some of the old damage/cracks & the solder used for the liner insert and repair can still be seen around the edges of the plate and within the cross hatched area engraved around the plate.
Just my guess of course.
*The frame also has an added screw high on the upper rear edge left side like an over hanging sear action boxlock. None added on the right side though.
* The hammer screw on the left side (rifle bbl) doesn't look original from the standpoint of it appearing flat topped. But that could just be the picture.
All of the frame screws on the left side have been worked over/removed,ect.
Those on the right side look untouched. That extra screw almost looks like a trigger plate screw head size.
Just some observations.