Justin Fishner-Wolfson, who runs a small investment firm above a tailor shop in San Francisco, is at the helm of a multibillion-dollar windfall.

The winners of SpaceX's blockbuster IPO—valuing the company at $1.75 trillion—might not just be investors, but also the ones who welded the rockets.

Justin Fishner-Wolfson, who runs a small investment firm above a tailor shop in San Francisco, is at the helm of a multibillion-dollar windfall.