mast

In England, at least as early as the 16th century – 18th(?) century, a unit of mass equal to 2½ troy pounds, about 933.1 grams, used for amber, coral, and gold and silver.

Source

A mast of corall weying vj^li skant.

A mast of coral weighing 6 pounds scant.

Will of Somerset 1502

Doesn't agree with later references. OED speculates that “mass” was intended.

Corralle the mast that ys to say [no amount of duty given]
...
Gowlde or sylver of Bruges the mast that ys to say viii s.

From a 1732 copy (British Museum Add. Roll, 16577) of a manuscript by T. Forgon, internally dated 15 July 1507, consisting of a list of customs duties on various articles, as reproduced as Appendix C in Norman Scott Brien Gras, The Early English Customs System, Cambridge (MA): Harvard University Press, 1918.

Troy weight hath seldom any greater denomination than the pound, yet sometime 2½ lb. thereof is called a Mast allowed for Amber and Gold and Silver Thread.

Samuel Jeake.
Logistikelogia, or arithmetick surveighed and reviewed....
London : Printed by J. R. and J. D. for Walter Kettilby ... and Richard Mount ..., 1696.

Amber --- the Mast containing 2½ Pounds...8 [shillings] 4 [pence] By 12 Car. II cap. 4 etc.

Henry Crouch.
A complete view of the British customs. 1st edition.
London: Printed by T. Baskett and by the assigns of R. Baskett, for T. Longman and T. Shewell, 1724.
Page 71.