Milton,
Not sure what code you are working to but this is from AWS D1.1,
6.17.12 Identification Marks. A radiograph identification mark and two location identification marks shall
be placed on the steel at each radiograph location. A corresponding radiograph identification mark and two
location identification marks, all of which shall show in the radiograph, shall be produced by placing lead numbers or letters, or both, over each of the identical identification and location marks made on the steel to provide a means for matching the developed radiograph to the weld.
C6.17.12 Identification Marks. This subsection describes all information required to identify the radiograph
and also provides methods for matching the radiograph to the weld joint, so that weld repairs, when
necessary, may be made without repetitive or unnecessarily large excavations. Radiograph identification marks and location identification marks shall be used to locate discontinuities requiring repair and to verify that unacceptable discontinuities have been repaired as demonstrated by the subsequent repair radiograph
If for example you have an A and a B shown on the radiograph then you should have an A and B marked on the part that has been radiographed.
Take the paper sleeve that the film came in (or other see through paper) and make what is called a "tracing".
Lay the paper on top of the radiograph while it is on the viewer and lightly copy/transfer the A, the B and the defect onto the tracing paper.
Now lay the paper on the part so the A and B match up and you can then mark the location of the defect.
If you can draw the size and shape of the defect accurately it greatly assists the person that is performing the excavation.
Hope that helps,
Cheers,
Shane