If I am wrong, I am sure one of the superintendents here like Pat K. will correct me, but ...
I don't think "South German" bent is a specific cultivar of bentgrass (such as A-4 or Penncross), but a MIXTURE of different creeping bents which was most popularly used in the 1920's. I believe there were several different bentgrass cultivars contained in it, including Washington bent which is know for its purplish color, and perhaps velvet bentgrass as well.
You do still see remnants of it at many old Northeastern courses, because their superintendents have not fallen prey to seed salesmen encouraging them to overseed with newer cultivars which would crowd out the older varieties.