"Washington" Pattern Ivory Set with Calvert-stamped Rooks

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A "Washington Pattern" ivory chess set, late-18th/early-19th century, one side stained red, the other side left natural, kings with cogged crowns, queens with ball finials, bishops with open mitres, knights as carved horses' heads, rooks as turrets signed: Calvert...189 Fleet Stt, pawns with ball finials and wide bases, the king 8.5cm high, the pawn 4cm high.

This set was sold in auction with the presumption the stamped Calvert rooks are replacements. Based on four separate trails of evidence, I am exploring the possibility that the rooks are actually original to the set, which would make this a very early Calvert set, possibly 1790-1800. If so, it would be a historical "missing link" for late 18th century Victorian sets.