Hydrographic Survey Station
Hydrographic survey stations are another style of pillar. They have no central column or plug, simply a Brass Plate. They are narrower than Hotine pillars. Ru Na Gavern http://trigpointing.uk/trig/5759 was used for the 1936 retriangulation and is shown on 1:50,000 OS mapping. They were however installed by the Admiralty, not the OS, and therefore have no Flush Brackets.
The term Hydro pillars can also refer to survey pillars installed by or for the Hydro Board for construction / monitoring of dams or other hydroelectric installations. For example three are mapped WSW of Loch Treig Dam which are shown as Pillars (Hydro) on 1:25,000 mapping, e.g. at NN 3373 7673. These are not hydrographic survey pillars - just pillars put in as part of a major civil engineering project and presumably by the contractor.
An apparent exception which may be OS built, is the pillar installed on Syart Law for the Megget Dam http://trigpointing.uk/trig/14724. This is of standard Hotine construction and has a standard spider, and may therefore be OS built There are no less than 5 trig symbols on 1:10,000 mapping, of which at least one more (non standard design) has been found on site. These are not shown on 1:25,000 mapping.