It came into use about the 14th Century in Britain when weights were beginning to be standardised at 16 ounces to the pound. In the current metric system, 100kg. The heavier standard which here has been termed refining weight was used on Dartmoor to allow for losses in final refining and a tax levied on that refining. In other words, it was a bit of a fiddle. The thousandweight used in some records sometimes refers to the standard pound and sometimes to the heaver pound.
Judgments have to be made which is in use in the document under consideration.