Published by Judges Guild.
A good case in point is 1978’s Ready Ref Sheets. This is in fact the expanded second edition of the product, the original having come out a year earlier. 56 pages in length, the Ready Ref Sheets, as its name implies, is a miscellany of charts, tables, and optional rules for use by referees of Dungeons & Dragons. The majority of this miscellany comes the first six issues of Judges Guild’s Guildmember subscription, supplemented by material from the Campaign Hexagon System booklet and other JG products. You’ll note that this edition is listed as “Volume I,” but, so far as I know, there was never a Volume II.
Read the Full Retrospective/Review at Grognardia.