1943 through 1964 Chevy made their own rear end.
Starting in 1965 Chevy (GM) bought their rear ends from Dana corp Spicer division. The Spicer ten bolt has an 8.2" ring gear and the 12 bolt has 8.875" ring gear; and all of the posi units were made by Eaton (posiTraction is a registered trade mark of Eaton Corp: which also makes the TruTrack, Detroit Locker and four other limited slip rear ends.
Starting in 1972 GM bough only one axle for all of their rear wheel drive cars from American Axle Works (the folks that make the 14 bolt rear end for three quarter and one ton trucks). That rear end called the "Corporate" because it was used across the whole fleet of GM cars has an 8.5" inch ring gear. AAW uses Borg-Warner limited slip rear ends. They eventually to make things cheaper changed the rear out for a 7.5" inch ring gear rear end that was first used by Chevrolet under the four cylinder Vega. When that proved to be too cheap, the factory raised it up to a 7.6 " ring gear. The 8.5" was reserved for cop cars and light trucks.
In 1994 Chevy brought back the Impala SS (basically a luxury interior cop car), and that car had n improved 8.5" rear end with a 30 spline axles (same size as the 12 bolt Spicer). So with a 1994-'99 8.5" rear end the only parts inside that are not as strong as the 12 bolt Spicer is the size of the ring gear. The 8.5" size has proven to be quiet strong (though I still prefer to use a Dana 60 with a 9.75" ring gear and 35 spline axles equipped with a Detroit Locker).
When Spicer first started making the rear end to GM specs they had a one inch NPT fill plug. By 1968 that had dropped down to a 7/8th inch NPT pipe plug (to save a fraction of cent on each car knowing they made 1.3 million cars a year). The AAW has a half inch pipe plug from the start. GM always starts out with an engineer designing the cars and car parts according to the designers goals. Before they are through with the first year's production the accounts are cutting costs on everything they can as that is how they justify their existence.
PosiTraction rear ends will usually have a metal tag on the right lower cover bolt with a two- or three-digit axle code and/or production date. The tag on the filler plug (usually white lettering on a red plastic tag) warns to use special lubricant additives for a PosiTraction and was attached to the pipe plug fill hole.