Rpg's method of checking for a loose fit is a good one.
Another very similar is to take the forend off.
Turn the gun upside down and hold it horitontal.
Then push the top lever to the open position and hold it open.
(The bbls won't fall off as the assembly is upside down)
With both your hands supporting the buttstock and frame only, nothing touching the bbls...
Shake the bbls gently side to side.
Don't flip them upwards,,just a gentle side to side shake.
If the bbls are off face any amt,,you will feel them move as you shake the assembly. Sometimes you can hear them rattle & click when they are really bad!
The weight and length of the bbls and the fact that they are hooked/pivoted up close to the standing breech in this simple test gives them plenty of leverage to show any amt of 'off the face' clearance looseness.
The bbls can also be off-face in spots around the breech but still be a tight fit at other spots.
This will make them feel tight, but gaps in the fit can sometimes be seen when held to a bright light, checked with feeler gauge matl, or the best way is to gently smoke the breech face of the bbls.
Then close and lock the bbls on the frame. Then open the bbls and check for contact betw the bbl face and frame,,or lack of it.
There quite possibly may be something less than a James Purdey shop fitting there. Things can be fixed w/o getting into welding and refitting a hinge pin and all that.
I think some of the pics are just showing the refinish rounded edges of both bbls and frame coming together and the shadow line it causes.
Check the chamber length vs gauge before any shooting if that is the plan. 1910 and before most any chamber length was the game.
That's the first Shattuck SxS I've seen in a very long time.
Single shot break open shotguns and small handguns,,but not SxS's.
The pic of the underside of the bbls shows two 'weep' holes drilled into the rib. One in front of the forend hook, the other into the 'short rib' behind it.
Those are generally drilled when bbls are aftermarket blued. They allow the boiling water that inevitably gets inside the bbl/rib cavity to get back out again. Soldered on ribs are never a perfect fit and the tiniest pin hole will let water in. It has no real way to get back out w/o these weep holes.
When orig mfg'd, the bbl assemblys are left open at the muzzle end and that is the massage point for water in,,water out. Then simply soft soldered closed after the finishing is done.
Never say never,,Winchester has always drilled weep holes on their Model 21 bbl assemblies when brand new. They too rust blued them. So they decided they were necessary.
Win plugged the holes after the bbls were blued by simply placing a lead shot pellet on the hole and lightly punching it into place. It flattens the BB and at the same time lightly rivets the shot into place in the hole.