This seemed to have been a rather common practice among electric utility companies here in the Boston area for many years, especially with Boston Edison. Perhaps the advantage was in providing safer and extra pole space on crowded pole tops. Or maybe for easier maintenance such as replacing fuses (note the rears of two "plug-in" style fuse cutouts on the upper arm). Of course a lower mounting height meant for easier transformer installation and maintenance too. Saugus is a city several miles north of Boston.