Oil switches like the one you see in between the two crossarms towards the left side of this pole were very common throughout the Boston Edison system.

Briefly...instead of switches such as solid-blade cutouts that operated higher voltage circuits in open air (typically 4,000 volts and above) these units did the job. The arcs from the switching operations were quenched in insulating oil which practically eliminated pitting, burns and corrosion on their contact blades that otherwise was commonplace with some open-air switches. Some enclosed types are surviving here and there to this day on the Edison system.

As an added note, Boston Edison always used glass pintype insulators and were purchasing them through the middle 1960s.. These were of the CD 164, 165, 165.1, 167 and other deep groove, double-petticoat glass styles. Many, many thousands are still in service and it is commonplace for the old glass to be transferred to the new pole's crossarm (for a second, third, fourth life...) during pole changeovers to this day...