Thanks so much. Could I ask what the source is? I did check your website, but had no luck! Don't have any of my usual resource books to hand at the moment so I'm a bit stuck in a rut!
The mark is illustrated in Douglas Bennett's book, Collecting Irish Silver, published by Souvenir Press, 1984. Walker's mark is one of many which I have not previously seen so have not had the opportunity to include on my site. May I use your image? and if so what is the mark on?