It is a doglock plate. The piece to the rear is the dog, it swivels and hooks into a notch on the hammer acting like a safety. It is a very early plate, but no way to know if it is to a blunderbuss. Blunderbuss were mostly used on ships or moving wagons, the belled muzzle was to aid in loading when on a moving ship or wagon. More likely to an early musket or trade gun they were all much more common than the blunderbuss. That is one big lock!