This is an insulator that I have had in my collection for over 20 years; a very rare one of a kind piece made by Swedish manufacturer Rörstrand. Well, according to me it look very similar to the style you folks call an EC&M! The first time I saw pictures of those EC&M's I immediately saw that they looked similar to odd one in my collection! Like the EC&M it doesn't have an inner skirt, but unlike the EC&M's this one have a top groove. The groove itself is straight, but the upper part of it is curved, and thus preventing the wire to come off when it is tensioned. One can wonder which is the oldest; the EC&M's or this one? But for sure there must have been some interchanging of insulator designs in those ol' days.