Actually Posi is what Eaton calls their clutch type limited slip differential that was used in the Chevy 8.2 10 bolt and 12 bolt in the '60s and early '70s. Eaton has a trade mark on the name Posi. The name was so popular that people started using it for all limited slip differentials, the way Asprin was used for pain killers and Roller Blade is used for inline skates. Governor Lock is also a Eaton differential that is used in GM trucks.
The 9 inch Ford Trac Lock is also a clutch type limited slip differential. Most of them have 4 pinion gears, but the early ones had 2 pinion gears. They use a clutch stack in the left side of the differential and a block with 4 small coil springs between the axle gears for preload. The clutch plates are lined with an organic friction material and are the same as the C4 transmission clutch plates, with the exception of the plate that goes against the clutch hub. The axle splines on the left side are split between the axle gear and the clutch hub. The differential cover on the left side is weak and is usually the first thing to fail. I've seen many of these that were broke by stock engines with automatic transmissions.