For anyone else stumbling across this thread, I found this:
From the back of a packet of 1oz. BWS by Brewcraft of Portland,OR, USA:
Contains Calcium Sulfate, Potassium Chloride and Magnesium Sulfate. Used to duplicate the famous Burton-on-Trent brewing water, known for it's [sic] pale ales. Use 1 tsp. per gal. to increase hardness of water by 889ppm. Add prior to boil.
Breakdown of the water hardness: 135ppm Ca++, 388ppm So4--, 19pp Mg++, 183ppm K+ and 165ppm Cl-.
Batch: 03089
Reorder Code: 990-A-1