This came up recently in a discussion of extract brewing and water treatment. No one seemed to have the answer offhand so I emailed Briess and asked the question. They were kind enough to reply with the following information:
"We do not regularly test our products for these salts, but we do
occasionally test the water used in our mash. Sulfate is around 63mg/L
and Chloride around 28 mg/L. Since we produce 16 Plato wort (Gravity
1.064), you can calculate that for every 1 Plato increase (Gravity .004)
from extract you use you will be adding approximately SO4=4mg/L and
Cl=1.8mg/L to your brew. The water that we use is of moderate hardness,
but since we make high gravity brews, the hardness will be lower if you
brew a beer below 1.064 using distilled water."
I thought this was good information and wanted to share.