Here are some more examples of the the Walker Knowles & Co mark with the extra marks appearing as "makers marks" . One auction house suggested an "E.A" mark could be a "tally" like worker's mark, that we do see on lots of on solid silver items in the 19th century period. As well as the 2 items with "E.A" we have 2 "I.B" (I think) marked items.
Could there have been outsourcing? It would be interesting to get some other views or information from Forum members.
Here is a note of caution regarding the cross and orb / circle mark. In the composite image below, the mark, not in a cartouche, is identified as the mark of another "Old Sheffield Plate" concern, Blagden Hodgson & Co, circa 1820-1833, of Nursery Street, Sheffield.
Items from the USA can be currently seen for sale, with the "Blagden" mark and proclaiming 1820's manufacture but with an additional "Made In England" notation. I can not remember exactly but I believe "Made In England" was an early 20th addition for items for export. Blagden Hodgson & Co is suggested as having ceased in 1833 and absorbed into Hawksworth, Eyre & Co along with their cross & orb mark and later it was said "H&E&Co" had a considerable export trade with the USA". After 1932 Ellis and Co (of Birmingham UK) took control of the firm. It may well be many items attributed to "Blagden" are by the 2 later concern owners of that non cartouche mark and are being mis-dated. Checking around on line, Just prior to posting this, I found another similar opinion referenceable.
I think this mark ought to be an early 19th century "Blagden Hodgson & Co" mark
Fishless