A rivet is a metal bolt or pin, with a head shaped like a mushroom’s cap; it’s used to fasten together plates of hard material, and thus is familiar to armorers. It is shown in its “pre-flattened” state, with the head to chief by default, as in the illustration. Though the term itself is period, we’ve no explicit documentation of the form registered in the Society; certainly the rivet is not to be found in mundane heraldry. Pending documentation of this form, the rivet will not be accepted as a Society charge.
William the Frogge bears: Per chevron purpure and argent, three rivets Or and a frog sejant affronty vert.