The year letters used in Amsterdam during the guild period (ancien régime) for so called
Grote Keur or 1st Standard were always in a round shield, never in a square shield. Also period 1663-1807 in combination with the lion rampant, the province of Holland 1st standard mark. Or since 1734 Amsterdam combined town mark and year letter attached (various shield shapes) for 2nd standard silver.
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In short, false hallmarks. See K. A. Citroen, Valse Zilvermerken in Nederland # 429 imitation of a master's mark, # 387 imitation of a year letter, and imitation of an Amsterdam town mark.
Peter.