The thumb hole lever operates a Purdey bolt, which is a perfectly secure method of holding the gun shut, whether operated by a thumb lever, a side lever or a top-lever. Early guns used a single Purdey bolt rather than a double one. Still sound.
If the gun (originally with 2 1/2" chambers has been lengthened to 3" it will be out of proof and valueless outside the US.
One must wonder what kind of abuse it has suffered with some bozo firing 3" American shells in a mid-weight light framed game gun.
The forcing cones will also be affected and it sounds like the gun has been spoiled . All too common with English guns in the US.
