I read somewhere that several stamps are used at the same time. For example, we have 10 stamps, each stamp has its own small differences from the beginning + there is some wear + a different force of blows from each smith (for example, for every smith 3 the force of impact (morning, afternoon, evening)) + the effect of subsequent processing (also we allow 3 variants of the difference). In total we have 90 variations end stamp. This is all a mental example and on the real business it can be more variations (and I think surely more)