D-421SP

The D-421SP is a specialized sports variant built on the D-321 platform that added full waterproofing through rubber gaskets, an O-ring-sealed lid, and protected external ports inside a bright yellow ruggedized shell. Underneath the casing it still carried the same general sound and ESP logic as the base model, but the machine was clearly being pushed toward a much less sheltered kind of use.

Portable CD was being pushed into genuinely active environments by then instead of just protected indoor use. The D-421SP only really makes sense once anti-skip had matured enough to support that ambition, because without it the whole idea would have collapsed immediately. It is one of the first Discman models built around exposure instead of protection.