Here is a link to the calculator forensics page, http://forensics.calcinfo.com/, which has a discussion about the algorithm and results tables for numerous calculators.
A few weeks ago, I was looking at Chinese calculator manufacturer web sites, and stumbled across who I believe is the manufacturer of the HP6S. I believe it is Karce (http://www.karce.com/). Here are their product pages for two calculators that, except for some color differences, look suspiciously like the HP6S Solar and HP6S.