Welcome to the Forum.
I do not know the answer to your question, but can offer a speculative guess as to the reason of the marking.
If the item was destined for export, likely to the States with your location, there would be no reason to bear the additional costs and delays of assaying and hallmarking as they were not required in that country, only that the item should be marked as 'Sterling'.
As to the 'D', Harrison Bros & Howson's New York agent was William Davidson, of 66, West Broadway. Could it be that the goods sent to Davidson were marked thus?:
Harrison Bros. & Howson - New York - 1899
As stated, just a speculative guess.