Push Rivets, aka Pine Tree Rivets. 13/16 in. long, ¾ in. head. Round ridges, 5/16 diameter shank, pointed end. Common on Old Town kayak seats (part number 01.1315.3590), also Perception, Mad River and Wilderness Systems kayaks. Used for Tarpon and Mad River seat pads, net side pockets on Search kayaks and the Phase3 Seating. Will fit into a .25 in. drilled hole. (alt Confluence part numbers; 9800255 Harmomy Plastic Push Rivet 0.75 in. Diameter - 6 pack, or 01-1315-3590 and 3140-0101, ITW 39063010-C3004D)
Push Rivets are typically used for a pre molded or pre drilled holes. They are used for seat pads, and other kayak features like side net bags on canoes and sit-on kayaks, as well as thigh pads on sit-in kayaks. The ridges on the shank will fit very snuggly into the hole and resist removal. Insertion is easy. Removal may require some prying and pulling with hand tools and probably damage the rivet.