I believe these are later 30s or very early 1940s and preceeded the otherwise identical alunimum ones that are seen all over former Boston Edison territory (they were bought out by NStar Electric about 10 years ago). FYI Boston Edison came into existance in the mid-1930s. Formerly the utility was known as the Edison Electric Illuminating Company of Boston.

With their age and the market price of copper nowadays there probably aren't too many of these left...