"Both chambers measure a full 3" to the forcing cone. ???? Could they be original ????"
This is a major concern. Many makers bored 2 5/8" 12g chambers at the turn-of-the-century, Hunter Arms being an exception. Some trap guns had 2 7/8" chambers for the longer shells (more wadding;
not more shot) of that era. This appears to be a bird gun however, and Cylinder/Full was a not uncommon combination.
You have no way of knowing what boomers have been put through that gun in the past.
If you don't have a bore scope and a way to measure wall thickness, walk away.
The result of a lengthened chamber (with visible tool marks) and a likely double powder reload
