I think Peterb is right. I think the original (x1?) had one conductor/wire encased in dielectric and wrapped in shield/drain. The x2 had two conductors/wires to pass both directions of the signal (drive & drain), with shielding wrapped around the two -- for single ended (RCA) connections. The x3 had three conductors wrapped in shielding, appropriate for balanced (XLR) connections. You wouldn't expect to see x2 used for balanced or x3 used for single ended interconnects.
