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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. But see the comment on Crouch, source 5.

sources

A mast of corall weying vj^li skant.

A mast of coral weighing 6 pounds scant.

Will of Somerset 1502

The magnitude doesn't agree with later references. The Oxford English Dictionary 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.

Amber…the mast conteyning ij li. ½…
Gold and Silver thred counterfeite voc. Cullen gold & silver the mast conteyning two pound & a halfe at twelve ounces to the pound.

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.
Page 184, page 191.

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 to contain 2½ Pounds … [There follows a computing of the duty on 6 casks of amber on strings. The shipper declared 6 casks containing 500 masts of amber. The officials compute 617.6 masts.] … When Amber is imported in large Lumps, it is not strung.

Henry Crouch.
A Complete View of the British Customs. Part the Second.
London: Printed for John Osborn and Thomas Longman at the Ship in Pater-noster-Row, 1728.
Page 20.
Crouch's computation is done entirely in avoirdupois pounds, including the size assigned the mast. If this example represents real practice, then by the early 18ᵗʰ century the mast was 2½ avoirdupois pounds, not 2½ troy pounds.