OWD: I can't answer your question as to whether that mechanism is the first for self-opener but I can say what a cool gun! The mechanism looks so simple. I wonder though if the cuts made in the frame to accomodate the self opener don't weaken it. Geoffrey Boothroyd in his book 'Sidelocks and Boxlocks' stated Tolley in his opinion couldn't be classed as a 'great' gunmaker but did deserve a place in the historical record. If Tolley was the first to invent the self opening mechanism then I think in this instance Boothroyd may have been wrong in terms of Tolley and their 'greatness'!
