Yes I'm very happy to be among like-minded individuals here on this forum, many of whom I consider dear personal friends. I ought to add,and it's difficult to do sometimes as is the case with "SL" but it's a safe idea to research the name further just to add that little extra reassurance.
Now, about this current mark: these are quite small. Is it a London assay mark? The books are arranged by assay office. I own Chester, and Sheffield through 1907, and I walk a short walk from work to the University library to consult the London books. To my knowledge a Birmingham book has not been made. If it London I can take a little walk on Monday and report back. Cheers!
Edit: occasionally London firms will register in Chester and appear in the book but that was not the case this time. I'll reference the London edition on Monday, if those are in fact London marks. Can't quite make them out.