Due to the fact that overhead wire is being used, rather than third rail, this must be somewhere on the South End. The North End also did not use block signaling, except in a few limited places. According to Timetable 28, dated 1939, the only 3-light signal on the SN was near Broderick, controlling the River Spur. The top signal controlled M St., the middle signal controlled Front St., and the lower signal controlled the River Spur. While I can not confirm that this is where the photo was taken, it would make sense, given the signal in the photo.

The multiple signal mast was a give away. The only location I am aware of that the S.N. passed such was at Broderick in West Sacramento near the M St. (Tower bridge).