the test metal contains iron, so it’s magnetic. Detectors are typically checked with three types of test metal: ferrous, non-ferrous like brass, and stainless steel, since each type behaves differently in the detector. Ferrous is generally the easiest to catch, which is exactly why it’s part of a standard test set.
The “25mm” most likely describes the size of the disc or carrier, not the metal sphere buried inside it. The sensing sphere is usually far smaller and marked separately in millimeters.
The cert number, that 213026B, is a traceability code. It ties the piece back to its calibration certificate, so a factory can prove the object was made and verified to a known standard. That’s what makes the testing auditable. Put all three together and there’s no ambiguity: this is a piece of quality-control kit, not a bonus ingredient. If you enjoy the puzzle of unlabeled objects, the same
