That's 1/16 diameter lead free rosin core solder, which you can pick up in the tool department at any Home Depot. It's wrapped on a 90 degree Owner cutting point saltwater jig hook I sell, along with the plastic baits of course
. The solder gives you some casting weight, but more importantly a slow spiral fall if you kill your retreive and allow the bait to sink. It won't sike quickly like a stone as would a similar bait rigged on a jig head, more like the real thing. Here's a link to an article I wrote on rigging and fishing the baits
www.ledgerunnerbaits.com/striper.htm. Many guys on the board here can attest to how much stripers love that bait and when they are keyed in on peanuts, nothing short of the real thing can touch this bait.