Here's a sample of the various types of threads found on insulator pins. Starting from the left, the first is a cast iron thread. These could be a solid pour or hollow and sometimes with a slot for a felt cushion. Next, a hard rubber thread poured in place on the steel shaft. Next, a cast in place lead thread, Next, a stamped metal thread made in two halves and arc-welded to the metal shaft. Next are the spring steel threads attached to a special base integral to the pin shaft followed by the threads of the carrier style insulator. These threads had a thimble of lead placed over the threads before the insulator was installed. These thimbles were also made of plastic. Next, the threads of a metal pin designed to take a wooden cob. the last pin shows the threaded wooden cob in place on a metal pin. There are numerous types of insulator mounting pins and brackets but most of them will incorporate one of the threading styles shown here.