In 1969 British librarians Derek Austin and Peter Butcher issued PRECIS: A rotated subject index system, published by the Council of the British National Bibliography. This appears to be the first published report on an innovative method for adding subject data in the form of descriptors to the computerized MARC record. The 87-page report with a theoretical discussion and many specific examples was followed by an undated 17-page "Supplement to "PRECIS - A Rotated Subject Index System." The new system was applied to the British National Bibliography.
Austin followed the 1969 report with an expanded book entitled PRECIS: A manual of concept analysis and subject indexing (1974). An expanded version of this was issued by the British Library Bibliographic Services Division in 1984. I have reviewed copies of the 1969 and 1984 publications.
According to a quotation from Austin's obituary quoted in the Wikipedia article on Derek Austin, which I accessed in October 2016, Austin's "aim was to create an indexing system that would liberate indexers from the constraints of 'relative significance' (main entries). ...As by-products of his indexing theories he worked out drafts that in the mid-1980s were accepted as British and International Standards for examining documents, and for establishing multilingual and monolingual thesauri". PRECIS was an example of the application of syntactical devices in indexing. It was replaced at the British National Biography by COMPASS in 1996, which was later replaced by Library of Congress Subject Headings.