OK, here is a brief historical bit:
305A is just a Bell System general purpose enclosure that was made out of stamped aluminium until the 1970's. It was drilled and tapped for mounting of a variety of protectors. The 305A only had room for one protector, either a 123A (one pair) or 128A (two pair).
The older drop wire that was mentined is standard 18.5 gauge standard parallel drop wire, which is perfectly suitable to support DSL. It consists ot two copper-coated steel wires laid in a parallel plastic or neoprene rubber jacket. It is similar in appearance to heavy-duty lamp cord.
The three conductor wiring was very common for station cable runs in the 1950's. The yellow conductor was used for a ground. If you were on a party line, the "A" party had their ringers connected between the ring conductor (red) and the yellow. The "B" party was connected between the tip conductor (green) and the yellow.
You won't see this, or the need for it anymore since party lines are long gone.
Oh, by the way, ground start trunks are still very heaviliy used in the US for PBX's. They won't be fading off into the sunset too soon.
[This message has been edited by EV607797 (edited 03-05-2007).]
