Yep this is a group relay, but nope its not series! All of Long Beach's street lights run on group relay systems. But look at the way the wiring is set up, can you tell what voltage this is?
Yep you guys are correct it is indeed 120v! If all three wires from the transformer was used, then it'd be 240v, but you can see one of the leads is not used. However, LB has some 240v systems too, I will post a pic of an example within the week.
Yep that tap is disconnected and not in use, but looks like its tied into place on the bottom of the transformer. BTW, the primary lines here are 12kv.
I wonder why they even bother to connect a wire to that tap? It wouldn't of hurt to just leave the tap bare. Of the could of just used a 120 volt transformer.
Most likely was used as it is a regular stock item. A single 120VAC unit would be a special order item, and having the spare secondary means you can use it if the other winding goes open, without replacing the transformer.
Making Us Guess in "Comments" Instead of putting it in the Description.