Dang, I even have them; I but had forgotten about them.
In 'Nam, we wired with an eye to the future. We used three conductor (12-3 w/g) wires to each junction/receptacle box. So 240V or split 120V phase circuits could be employed later by simply tying in the second conductors to added breakers at the main and to outlets at the local junction/receptacle. One can convert a duplex receptacle to split phase by simply breaking off the tandem socket conductor tab on the receptacle unit, thus bringing in the output of two separate breakers into the single receptacle. Or; the receptacle can be replaced with a crowfoot to provide 220.
The hardest part of adding circuits or distributing 220 within an existing structure is the addition of the necessary conductors. By anticipating the need during initial construction, cost is added up front, but more is saved on the back end. The cost of materials during construction is far less than that of the labor needed to install them, regardless of which path one chooses. This way the wires are already spare and waiting, with no more stringing needed. Pay me now, or pay me much more later.
The key exception was that any aluminum conductors must be immediately torn out and replaced upon discovery.
There are other ways of doing this for single phase with existing wiring (if they are composed of a continuous path of conducting armored shield wire (like BX) and conductor tagging is essential); but modern codes wisely prohibit their use. Unskilled tinkerers could end up with an electrical disaster on their hands.
We tagged conductors within the main box with destinations. Site maps and conductor tags were inscribed on the obverse of the breaker panel shield.
USMC Engineer Electrician School was always ahead of its day. The continuing training concept was colored by the precept that a jack of many trades must also be a master of all of them; with a promotion scheme that paralleled the apprentice, journeyman, master progression. I was an E-4, which corresponded to a master. E-5 was organic unit instructor, E-6 was contractor (ours was a WO-2).
Good call.
Greg