according to ISO 898, part 1
Marking is obligatory for property classes of or higher than 5.6 and is preferably to be made on the threaded part by an indentation. For adjustment bolts with locking, the marking must be on the side of the nut. Marking is required for bolts of nominal diameter of or greater than 5 mm.
The symbols shown in the table below are also authorised as a method of identification.