Flush brackets were first used during the 2nd Geodetic Levelling of England & Wales between 1912 and 1921. The 2GL series were numbered from 1 to 3000 and did not have a prefix letter. The vast majority of the flush brackets in the 2GL series were placed on walls and buildings, but a small number were used on triangulation pillars. There are 46 examples of these listed in the database, all in the range 2943 to 2999.
In addition to England and Wales, the survey also covered part of the southeast of Scotland, in roughly the area between Newcastleton, Dunbar, and Berwick upon Tweed.
The S-series flush brackets were introduced in the 1920s, and appear on both walls and triangulation pillars. As well as flush brackets S01 to S9999, the series also includes brackets numbered 10000 and above, even though these flush brackets do not carry an S prefix.
Within the S-series there are a number of distinct styles. Brackets S01 to S1134 have the S below the number, rather than as a prefix. Brackets in the range S3200 to S3677 have the S prefix above the number. These are sometimes referred to as BsM brackets. Numbers between S01 and S0999 all have a leading zero before the number.
The lowest S-number on a triangulation pillar is S1268, which means that there are no pillars with the S below the number. Conversely, the BsM style of flush bracket appears mainly on triangulation pillars, with currently only 7 examples of BsM wall brackets in the database.
The G-series flush brackets first appeared at the start of the Second Geodetic Levelling of Scotland in 1936. Unlike the S-series, numbers below 1000 do not have a leading zero. On most G brackets the letter appears as a prefix, but for a small range from G1000 to G1099 the G appears above the number. The G-series flush brackets are found only on walls, never on triangulation pillars.
The L-series consisted of just sixteen flush brackets, numbered L1 to L16. They were installed during the re-levelling of Greater London in the early 1930s.