As I time on my hands yesterday, I compiled a montage of number of marks used throughout the second half of the 19th century, all of which have been attributed by a dealer, auction house or other seller to John Smyth.

Photos 1 (1852) and 2 (1855 - the year John Smyth is said to have ceased working) show one of John Smyth's known maker's marks, the quatrefoil J S mark as portrayed in Douglas Bennett's book "Collecting Irish silver", although the shape is more like Bennett's mark 305 but without the dot than Bennett's mark 306. Photo 3 (1858) shows that his son(s) continued to use this mark. It is interesting to note that this mark was still in use as late as 1870 as seen in Photo 7. However, I could not find any silver with it during the 1860-1869 decade, which isn't to say such silver does not exist, just that I could not find any examples online of John Smyth's work showing the maker's mark for those years.
The first example I could trace of the J • S maker's mark in a clipped rectangle (same as Diane's 1867 spoon shown in Photo 5) dates to 1862 (Photo 4) and it continued through 1868 (photo 6) until 1872 (Photo 8). In 1873, another known mark of John Smyth appears, that of J.S in a clipped rectangle and it continues for many decades (see photo below and Photos 9 & 10 above).

If it weren't for the 1870 J S quatrefoil mark (Photo 7) and the fact that J • S in a clipped rectangle appears before and after it, one could surmise that John Smyth used this J • S mark during the 1860s before changing to J.S in a clipped rectangle in 1873. However, the use of the quatrefoil-shaped mark in 1870 casts some doubt on this. If anyone can trace other silver by John Smith dating to the 1860s with the quatrefoil J S mark or any other mark, please post.
Websites that display Irish silver makers' marks and portray the J • S mark, including the 925-1000 Forum, show it as unattributed. So, was it used by John Smyth and if not, who was this maker?