Here is a neat ad for the Benjamine Cluster Sockets. They are mostly found in 2 socket applications but I have seen ones up to 6 sockets. And all are pretty pricey too. This ad is also from a 1921 catalog. There is a great website archives.org that has all kinds of old catalogs on it. I've found that the end of the year catalogs and the begining next year catalogs have the most ads but it's always good to check every issue because you never know what might be slipped in. It's time consuming but worth it to find a rare ad or information.