I can truthfully say that I have handled a thousand 4 pin tramp brackets over the last few years. I have an accurate count because I still have most of them. Canada seems to have used two types, in about equal numbers. In both types the base of the pin is threaded and nuts secure it to the frame. Today I noticed this bracket, still attatched to the crossarm. Identical to the other brackets except that all four pins, instead of threads have been rivited to the frame. How common / uncommon is this ?