Superb! Williams & Powell were at South Castle Street from 1834 to 1905 which is not much help in dating. The name still lives on and is owned by Chris Caine who has W.Richards Gunmakers; previously of Liverpool but now in Pocklington, Yorkshire. I do know that W&P records are lost. It might however be worth contacting the College of Arms
http://www.college-of-arms.gov.uk/ to see if they can tell you to whom the crest was granted. It might put some flesh on the bones and provide a provenence. Good find. Lagopus.....
