This is the SP Studio Systems 24 x 36" Softbox for 150 and 250 series monolights.
The SP Studio Systems 150/250 Series Softboxes are available in 3 easy-to-assemble models. Each Soft Box slips over the front of the strobe and is secured in place with two locking screw knobs. The front panels, made of a white translucent cloth, provide a soft light, making them the ideal accessories with your main light source for softening the shadows in group portraits and large product shots. Each softbox comes with an extra interior diffusion panel to totally eliminate any hot spots and to cut down the amount of light in those situations in which the illumination would otherwise be too strong.
White diffusion combined with silver interior gives medium soft light.