Are there no other mark perhaps small standard mark, the so called sword mark? The spoons and most marks appear to be crudely cast.
I*S is an other pseudo mark imitating the maker's mark of the Amsterdam silversmith Jacob Schenk (1744)
Could you please show us the other marks, the marks on the first four spoons differ from the last two spoons shown.
Source; Valse Zilvermerken in Nederland K.A. Citroen