This one is for Colin Jung and others who have asked recently about double threaded insulators. Here are three CD 143's and a CD 145 GNW TEL CO for your perusal. From Left to Right, CD 143 No Embossing Canada , a small beehive that is the threaded version of the CD 743.1. It has a flange at the bottom of the upper wire ridge that is not plainly seen in this pic, but I may post a close-up of it at a later date. Next is your usual CD 143 Dwight Pattern, and then the CD 143 NEC  double threaded. It is quite easy to see the steeper angle that the threads are on when you put one of these beside other seemingly similar pieces! Last but not least is the CD 145 GNW TEL CO. Hope this helps!