there is a possibility it is a version of the "california three way" or a "french three way" or even a "Chicago three way".
I have heard various names for these two different wiring configurations, and there are probably more variations of them yet,
more than likely what they did was take a hot wire and wire it to the single side of the 3 way switch and then take the two runner/carriers wires and take them to the other 3 way switch and then to the bulb and then to a local neutral wire on a different circuit if all you can fine is two runner/carrier wires,
and there is another version of the wiring is where basically it feeds from two different locations or circuits on the same phase side, with the bulb in the center. and the three way switches (one each end) are either feeding the bulb two hots or two neutrals, and either way the bulb is off in that instance but once one switch is flipped to the other side it completes the circuit,
(this method was used on yard lights a lot at one time as you could get by with jsut one wire coming to the light post from each building).
It is not a suggested method of wiring a three way switch,