An archaic or dialectal plural of bilk, or possibly a plural form of a variant spelling in certain regional English dialects.
This appears to be a rare or non-standard plural form—'bilks' is the standard modern plural. The '-is' ending might reflect older English or dialectal variations.
Language changes over time—what seemed like normal plurals 300 years ago now look really odd to us, which is why you find weird old forms in antique texts!
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