Metric arriage bolts have a small square shoulder under the head that fits into a prepared square-bored hole. The self-locking square neck prevents rotation, holding the bolt in place while tightened with a mating nut.
A4 / 316 Stainless Steel fasteners are "marine grade" corrosion resistant; suitable for salt water applications. A4 fasteners also offer increased resistance to chemicals when compared with A2 stainless fasteners.
