This video shows the straight shaft version. The curl barbells are exactly the same other than the curled shaft.
Virgin Rubber Heads over a Steel Core
No cheap cast iron here. The heads are first constructed of solid steel that the knurled shaft is inserted into and welded into place. Then the heads are coated with an odorless virgin rubber. The virgin rubber is also nice and smooth so is easy to wipe down with a rag for occasional cleaning.