Remember, as far as Jobs was concerned in early 1983, the Twiggy FDD was a part of the final design of the product, so those Twiggy prototypes could've been nearly final. The Sony FDD cases were prototyped after the Twiggy drives were abandoned, which kicked the prototyping cycle way back. The Twiggy cases you've seen could've been close to final prototypes, but your Sony case could be much earlier in the process toward the final Sony case design.