Common and pretty cool device. Harken "Magic Box". It take the place of a small-block multi part tackle. Tidy and powerful way to tension a line. Not cheap, either.
ps: just checked the Harken site, and it looks like they no longer sell these. There are a few for sale on the 'net, used.
(Sic Transit Gloria Mundi)
Ahh, ok that gives me some direction. I just became the new owner of an ensign fully set up for racing and everything made sense when I was going through it but then there was that and I just wasn't sure what it was. Luckily the owner is very active in our club and very interested in showing me around the boat when I get it back in the water.
No... it's a 8:1 tackle, with the sheaves all contained within the frame of the box. Travel is limited to about 3" less than the length of the box because of the size of the sheaves. At the time it came out it was wonderful because the ball-bearing mounted sheaves made adjustments almost effortless. Now it seems to have a good bit of friction compared to newer, cascade-type setups.