To be clear, I think the type you are referring to are the later type. At the earliest, I think they arrived with the 3500S manual in around '72. Certainly they were pretty well standard by the end of production. They have the upper structure formed from two stainless steel rods, set around 3 to 4 mm apart. The Triumph stag used similar. They are not widely available today. I have seen sets at £60 a pair at an autojumble and one of the Triumph Stag specialists sells them - but that is all I've found. I believe the idea is that they are not as subject to lifting off the screen at speed.
Chris