This is an unusual "buck-arming" or double-arming bracket that was used to secure two crossarms mounted on each side of a pole. I've never seen these anywhere before other than on the Hingham MA Light Plant system. By the time they were converting the entire town to 13.8 kv in the late 1980s there were very few of these left and I was fortunate to have gotten a few sets.

The last image (Photo 4) for more information. I am posting three more views of this bracket. The additional photos can be seen by clicking "Previous" after each one.