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mount

In England, 15ᵗʰ – 17ᵗʰ centuries, a unit of mass used for plaster, = 30 hundredweights.

examples

23 May [1480]. From the ship of William Philpot called Clement of London.
John Elyot, A[lien], 8 millstones, 12 panniers of glass, 2 mounts of plaster, 1 puncheon vine shoots, 3 tin crosses, £18 16s 8d.

H. S. Cobb, editor.
The Overseas Trade of London. Exchequer Customs Accounts 1480-1.
London Record Society, 1990.
Page 39, entry 131.

Plaister of Paris the mount conteyning three thousand weight

“A Subsidy granted to the King of Tonnage and Poundage and other summes of Money payable upon Merchandize Exported and Imported.”
A statute from the 12th year of Charles II, 1660. The selection is from the Booke of Rates, which is not part of the statute proper but developed from it. Both are printed in:
Statutes of the Realm, Volume 5: 1628-80, John Raithby, editor.
London: 1819.