Short answers, pulled from the story.
The Territory of the Saar Basin was a region governed jointly by the United Kingdom and France from 1920 to 1935 under a League of Nations mandate. Its capital was Saarbrücken, and its population reached 812,000 people by 1933. The territory closely matched the modern German state of Saarland, though it covered slightly less area.
The Territory of the Saar Basin returned to Germany after a plebiscite held on the 13th of January 1935. The League Council approved the reunion on the 17th of January 1935, and Germany formally re-integrated the region on the 1st of March 1935.
A five-person Governing Commission, known as the Reko, governed the Territory of the Saar Basin under the Treaty of Versailles. Its first chairman was French; later chairmen included George Washington Stephens of Canada, Sir Ernest Wilton of the United Kingdom, and Sir Geoffrey George Knox, also British.
With voter turnout at 98%, 90.8% chose to rejoin the German Reich, 8.8% chose to keep the existing mandate status quo, and 0.4% chose union with France.
The Territory of the Saar Basin issued its own postage stamps and its own currency, called the Saar franc.
After World War Two, a French protectorate was established over the region within borders similar to those of the Territory of the Saar Basin. A referendum on the 23rd of October 1955 ended French rule, and on the 1st of January 1957 the Saarland joined West Germany.