The brown urethane parts shown above are unused, unpainted hardcopy parts for an alternate version of the Kalibak action figure. The casting material used on these parts is known as carbalon. Dynacast, a green colored urethane, was more commonly used during the time period of the Super Powers line, but hardcopies exist in both materials.
The comparison shot shown above showcases the hardcopy parts next to their parent, the original wax sculpting. To learn more about this sculpting and this alternate version of Kalibak in general, I recommend reading the entry located here on the site.