Many, but not all bolts, are stamped with either a number (metric bolts) or a series of lines (fractional or "American" bolts) indicating their minimum strength or, more technically, their property class. Besides, the head markings' most reputable manufacturers also stamp their trademark in the bolt head so that it is readily identifiable and traceable. Why aren't all bolts stamped? The laws regarding the manufacture of hardware are pretty vague, and there are lots of ways to get around them. For example, hardware manufactured to OEM specification isn't required to be marked. That's why there are no indicators stamped into the head of your swingarm bolt. In another example, if a hardware manufacturer claims his high zoot chrome plated stuff meets or exceeds a particular OEM standard and says so in writing, he doesn't need to actually stamp the head of a bolt.