Sure, in Old English, "byldaþ" was used for the plural. A relic of this seems to even be present in Middle English (the following shows two forms though, I guess showing that the language was changing):

Furst þay [bees] bulden þe kynges hous..and þerafter þay buldeþ oþer hous.

(From John de Trevisa's translation of Bartholomew de Glanville's De Proprietatibus Rerum)

However, in the context of the quote on the dam, it looks to me more like hypercorrection, wording created by people who knew that archaic English sometimes used -eth but wasn't really familiar with it. After all, Shakespeare's English is the archaic English that's typically emulated, not the English centuries before that.

Sure, in Old English, "byldaþ" was used for the plural (þ being thorn, the old way to write th). A relic of this seems to even be present in Middle English (the following shows two forms though, I guess showing that the language was changing):

Furst þay [bees] bulden þe kynges hous..and þerafter þay buldeþ oþer hous.

(From John de Trevisa's translation of Bartholomew de Glanville's De Proprietatibus Rerum, a1398)

However, in the context of the quote on the dam, it looks to me more like hypercorrection, wording created by people who knew that archaic English sometimes used -eth but weren't really familiar with it. After all, Shakespeare's English is the archaic English that's typically emulated, not the English centuries before that.