The Society of Indexers (SI) is a professional society of indexers based in the UK, with its offices in Sheffield, England, but has members worldwide. The society was established in 1957, while its quarterly journal, The Indexer has been published since 1958. History. The Society of Indexers was formally constituted at the premises of the National Book League in the UK on 30 March 1957 by G. Norman Knight and approximately 60 other people.
https://en.wikipedia.org/wiki/Society_of_Indexers