I have a sugar shovel with identical makers mark, dated 1870. I have seen no makers mark recorded similar to this one because the S is so obviously smaller than the J, so it is still listed by me as maker unknown. The following is offered in comment on makers you mention
I would discount John Smyth as he is said to have worked 1827 - 1855 and his marks I have seen are not identical
The only mark for John Scriber I know of is IS
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Larkfield - Do you know where I might find information on the Scriber you mentioned? I sent an e-mail to the Dublin Assay address listed under the Dublin Castle website as a starting point.
I'm not sure if this will help or not, but "Nil Durum Volenti" is the motto of the McKnight family. The lion is also found on the McKnight family crest. I know it isn't much, but hope it can help.