Secondary Block Number
This number - in the form of SU30 - shows in which secondary block the trig station falls. The blocks were formed by joining up the primary trig stations to form polygons (trig blocks). Each polygon had a unique number (secondary block number). Within each block each trig station was named and numbered but not computed. These station numbers and names are unique within each block.
This number, in the form of /020 or /T20, shows the station number and its order (accuracy). A straight / and number show the station was a secondary station. A /T and a number shows the station was fixed as a tertiary or fourth-order station. A station number preceded by /H show the station was emplaced by the UK Hydrographic Office and fixed to fourth-order standards.
A station number preceded by /DM show the station was a 'distomat' observed station, fixed to fourth-order standards. The distomat stations were only fixed by observing distances and not angles, unlike other stations that were fixed by angle and distance methods. There are stations that have combinations of these numbers, for example, HU22/DM/T012.
A secondary block number and /Int and a station number indicate that the station is an intersected point. These are 'unoccupiable' points that were fixed by intersection only, for example, chimney or church spire