One of the original insulators on a holder. Notice the 'shoulder' on the side of the insulator. There had been a duscussion about whether this 'shoulder' served to hold the wire before a lineman tied it to the insulator (not unlike the 'ears' on French 'gingerbread men') or it was to facilitate screwing the insulator on the pin. The decision initially was in favour of the latter hypothesis due to the fact the 'shoulder' on all the known insulators had a slight outwards slope and didn't seem to fit good for wire-holding purpose. I'd like to notice that properly screwing an insulator on a pin requires considerable amount of physocal force and sometimes special tools are used to tighten it. I've tried to install an insulator with this 'shoulder' and it is usefull indeed (intentionally or not). However, an insulator that you can see in the photo has been discovered and it shows that the 'shoulder' did serve the purpose akin to French insulator's 'ears' (though this doesn't completely cancel out the other hypothesis).