What is "trone stone"

Trone \Trone\, Trones \Trones\, n. [LL. trona, fr. L. trutina a balance; cf. Gr. ?.]

1. A steelyard. [Prov. Eng.]

2. A form of weighing machine for heavy wares, consisting of two horizontal bars crossing each other, beaked at the extremities, and supported by a wooden pillar. It is now mostly disused. [Scot.]
   --Jamieson.

   Trone stone, a weight equivalent to nineteen and a half pounds. [Scot.]

   Trone weight, a weight formerly used in Scotland, in which a pound varied from 21 to 28 ounces avoirdupois.

n. A weight formerly used in Scotland, approximately twenty pounds.