Hi Daryl,
The attribution of maker's marks on silver plated items is often a matter of supposition. Information linking the maker to the mark was most often found from old catalogues, advertisments, sometimes from examples held by companies still in business and sometimes from pattern books were the pattern was peculiar to one company, but sometimes identification is made on a 'most likely' basis.
With little regulation in that industry, especially in the mid-nineteenth century, firms blatantly mimicked the official hallmarking system, this was not an attempt to dupe the buyer, but purely a sales ploy, after all they were producing an imitiation of solid silver items and the mark was the final touch. This is of course exactly what the buyer wanted also, goods at a cheaper price, but when laid upon the dining table, a vast majority of the guests would not know the difference.
Returning to the 'most likely' indentification process. I do not know if the attribution of John Yates is backed up by something more concrete, it may well be, but considering your comments on the dating of other pieces found at that site, there are likely to be three scenarios, that the item was lost at a later date, or the dating of the working period of John Yates is suspect, or that there may possibily be a mis-attribution to John Yates. If the latter were to be the case, I would throw the name of John Yapp into the hat. The Birmingham company of John Yapp & Co. can be traced back to the days of Thomas Willmore in the 1770s, but as John Yapp & Co. to the 1840s, a date more fitting with other objects from this site, but as mentioned earlier, there may well be more solid information linking this mark to Yates than I am aware of.
As for links to information on other silver plate marks have you checked out?
http://www.925-1000.com/silverplate__menu.htmlRegards Trev.
