The "large" National Insulator Company "corkscrew" known as the CD 110.6 was known to have been used on fire alarm circuits in and possibly nearby Portsmouth, New Hampshire. Sorry but none in this picture but I thought I'd share some cool info.... These insulators also have been found in at least one or two other place(s) in the Northeast. I do not know when the Portsmouth CD 110.6 insulators were discovered nor by whom. I am very familiar with that city and they have had openwire fire alarm circuits mounted upon white=painted crossarms, some stencilled "FIRE ALARM" in darker letters. These typically were upon two-pin crossarms and some still can be seen as such around that community. I've never seen a CD 110.6 nor anything else unusual upon Portsmouth vicinity fire alarm circuits.