What is the trace on the surface of the round head slotted machine screws?

What is the trace on the surface of the round head slotted machine screws?: There are traces on the surface of the round head slotted machine screws. Generally, there is a problem with the mold. For example

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stainless flat head screws, surface a variety of color can do it?

What effect does washer head screws have?

Die Spotting Extra Long screw