442 instances of "OLD TRIG" occur in OSGB36 list. These are a subset of the triangulation marks that were used in the 19thC Principal Triangulation and which were then re-used for the 1936 Retriangulation.
Details as to the appearance of these marks is incomplete. For roof stations, features of church towers including centres, vanes and spires, were commonly used.
For ground stations, the mark was often a buried block of stone which might have a hole in it . An example is at Flinty Fell. In other cases, the mark would be a pivot benchmark or an arrow cut into rock.
In some cases, the block is located coincident to an OSGB36 pillar. For example, An Cruachan and Carn Gorm. In the latter case, the pillar has toppled but this has not exposed the old trig.