A rivet is a common type of triangulation mark.
There are 1404 instances in the OSGB36 list. Of these, 5 are Order 1, 85 Order 2, 348 Order 3, 928 Order 4, 2 Order 12 and 36 Order 13 (orders 12 and 13 denote GPS stations). There are 141 instances of rivets that have been used as passive stations some of which may been re-used from OSGB36.
OSGB rivets occur mainly as roof stations and are not readily accessible whilst passive station rivets are mostly ground stations. The marks are usually installed on pre-existing man-made structures. However, exceptions to these generalisations are readily found.
Rivets are sometimes used as Witness marks and this can can cause issues with identification where the trig mark is also a rivet, for example Saskills.
A triangular punch mark is commonly seen in rivet marks but this feature may not be specific to OS.