Series circuits operate on a constant current, which means everything operates on the same current. So it makes more sense to rate series lamps in amps rather than voltage. The lumens are used because street lighting standards often referred to lumens.
15A and 20A series systems also existed. In fact a bunch of different current circuits already existed at some point, but the three most popular to my knowledge were 6.6A, 15A and 20A.
What were/are the voltages of series circuts? or does it vary among circut? Does the voltage of the circut matter in terms of the load used as long as the load was designed for series?
Yes, while a parallel circuit has various currents, a series circuit has various voltages! Each lamp in the circuit takes the voltage it needs to reach a certain power. Small lamps will take a quite low voltage, and large lamps will deal with much higher voltages. But the current stays the same in every lamp of the circuit. This is a big advantage when it comes to the wire size needed. A 6.6A circuit can be wired with #14 AWG wire, no matter how long is the circuit, 10ft or 40 miles, as long as the voltage sent by the regulator is high enough.
Does the voltage affect what type of transformer is used or is it only the amperage that matters? Are series transformers still made today? Are series circuts all DC or are they AC and DC or just AC?
The series lighting system can be powered by AC circuit as well as DC circuit. Just like the miniature Christmas lights powered by 120 V 60 Hz AC circuit I doubt that the DC circuit would go far for the street lighting system
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I have two GE versions ans 2 x 6.6a 4000 Lumen Series Street Light Bulbs see HERE[/url and [url=http://www.freewebs.com/lamps019/GE-6-6a-4000Lumen-Street%20Lighting%20Lamp.jpg]HERE. if you do testt hem best using a 12 volt battery as they are low voltage lamps about 15 to 30 volts depending on the lamp size
All the Best
Colin