Clabrough was an Englishman who came to the US and established a few shops out West to sell guns that were basically made in Birmingham. His most notable shop was in San Francisco.
The guns themselves ranged from fairly prosaic, such as yours, to very high end. But they were all made with good English craftsmanship.
A new book has just been published on the Clabrough guns. "J. P. Clabrough Birmingham Gunmaker" by Lawrence Skelton. Cost is about $85 but is full of photos and information and fairly well done. Only available from the author at
http://clabrough.com/index.php?p=1_7...rough-Gunmaker
I recently had two Clabroughs, one a hammerless like yours. Nice guns.