The 5ALT-20R is actually in the NEC 2002 handbook under exhibit 406.2. It refers to NEMA WD-6. I can find no reference in the NEC as to when it is required.
I made a mistake. In NEMA Std WD6-1997, the “Figure 5-20 ALT” is labeled as Alternate Construction – Canada and has one horizontal-only slot {versus a “T-shaped” slot.} Looks like is designed to preclude insertion of a 15-ampere plug.