Telephone subscriber lines beneath. Most look like twisted pair wires. Such loops and short runs used this type of cloth-insulated cable until some time in the 20s when more durable wire for this purpose was introduced and thus used by the Bell System companies. The latter was much more durable and weather-resistant. It had two pararell wires (opposed to twisted ones) and were individually rubber covered with a durable cloth coating (or sheath).

Using the top groove for the line wire and utilizing the second much wider groove area beneath for handling more than one twisted pairs could have been the intent of the CD 102.4 N.E.T.& T. Co. design. Maybe that's one here :-)

This is a quite a rare instance of Hemingray Provo No. 2 insulators used around here. I think I have seen something like them in a couple other old Boston area photos that I'll be posting in the future. Thanks Dustin for the ID on these.