Special Project pillars
A few trig pillars were built not for the Retriangulation but for other projects, and aren't shown on 1:50:000 scale OS mapping. Some are associated with one-off projects, such as:
British Leyland Works, Cumbria http://trigpointing.uk/trig/7511 (FB 11392)
Zeneca, Verdley House, Sussex http://trigpointing.uk/trig/7421 (no FB)
Manor Valley http://trigpointing.uk/trig/25787 (no FB), Syart Law http://trigpointing.uk/trig/14724 (no FB) and Hunter Hill http://trigpointing.uk/trig/25808 (no FB), Scottish Borders, all apparently relating to the Megget Dam
Larger scale projects involving larger numbers of pillars, are Project Emily (dating from the Cold War), the Great Glen Project, and the Channel Tunnel (Castle Hill, Cheriton Hill, Farthingloe and Shakespeare Cliff).
One special project pillar which was incorporated into the OSGB network, is Shelton Bar http://trigpointing.uk/trig/5931 (FB 11556), built for the 1986 Stoke on Trent Garden Festival. It was the last OSGB trig pillar to be built, in 1985, and was computed in 1987.
In the 50's and 60's OS was keen to participate in scientific projects like the Great Glen experiment and others. The practice developed of using OS pillars as permanent, stable, marks for such work. Later, in 80's and 90's the OS didn't look favourably on using OS style pillars for such work, and the practice was discontinued. With a few exceptions, these pillars were not entered into the trig database - they are not trigs - they have no consistently computed coordinates with the OSGB36 triangulation network.