I'm sure the answer you're looking for is Zagato.
However, I don't think Zagato had anything to do with this car. Porsche wanted Zagato to do the work, but due to contractual conflicts, couldn't approach the carrozzeria directly. Instead, Porsche enlisted Carlo Abarth to act as the middleman. In the end, while Porsche believed Abarth was contracting with Zagato, Abarth actially hired Scaglione to do the design and had the first prototype built by Viarenzo & Filliponi, not Zagato.
There are a number of sources to support this. Here's one from the Revs Institute:
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