I saw nothing on the Williams website. I strongly suspect they are using repackaged malts, like many other LHBS. There are only a limited number of companies that having the brewing facilities to make BREWERS GRADE extract, as opposed to FOOD GRADE, which in my opinion, does not give the same results. (think of products made for cupcakes instead of beer).
Briess is the largest extract brewery, and re packages a lot of extracts. Coopers is also a dedicated extract brewery, but I don't know if they repackage.
Short answer, if I think a malt is repackaged, I use values for Briess. I believe the generic values in BS2 are for Briess.
Ed