Personally I haven't seen differences in the pre SW (Rule & Level) and the SW blades, in terms of apparent structure (when grinding etc). I understand the SW mark was really a sub-brand marketing device relating to when the 2 companies merged around 1922 (I think?) and didn't imply specific technical changes. I mean the mark alone may not denote tech / spec differences, I don't mean there were none during the life of that mark.
Jim here found a good ID site for old US Stanleys a while ago, but it's somewhere in one of his boot fair threads and I can't find it.
Jim - find it again please??