I'd guess the top lever spring right off as well.
Weak replacement maybe, orig cracked.
But just a guess and you don't know till you get inside.
A century + of grim and built up crud slows things down as well.
A good clean & lube helps tremendously and that spring would need to come out in doing so. It's health can easily be seen and felt at that time.
The over center lockup (left of center) can be looked at too and a decision made on how best to tighten up the action.
Here's a couple pics of the inside .
The Top Lever Spring is removed in these pics. It compresses betw the short stud in the underside of the tang near the edge and the arm on the top lever spindle.
The small empty hole is the anchoring point for the spring,,a small peg on the bottom side of the spring at the V intersection fits into that hole.
..This gun appears to have had a replacement top lever spring fitted. There are two achoring point holes plainly visible. One, the original I assume, has had a small piece of lead
shot hammered into place to fill it most of the way.
The replacement spring was likely one that 'fit pretty close' and
need a new achoring hole drilled. Kinds close to the edge of the underside of the tang,,but it works OK!
The close inletting of the wood stock supports the spring when all is assembled so it doesn't work it's way out of position.
The locking bolt is seen bolted to the opposite side of the spinde crank and moves back and forth as the top lever swings.