Highly durable Walkman, with an aluminum cover (1 mm) and a plastic body up to 4 mm thick. Along the lines of the "WM-F5," this model was developed to withstand outdoor use. It is splashproof and impact-resistant. Its rugged styling emphasizes the durability of metal. A large die-cast aluminum fastener holds the cover securely in place. In operation, this was the first Walkman to use a C battery. By taking along a back-up rechargeable battery, users can enjoy remarkably long playback of 33 hours.