That was not a 12 bolt gear ratio.
If it was a valid number it would indicate a two series carrier which will restrict you to a two ratio gear set. If it where a 2.73 up to a 3.90 it used a three series carrier which means you can pick any valid three series factory gear ratio (3.07 or a 3.08, 3.42, 3.55, 3.73), with a four series carrier allowing you to choose any 4, 5 or 6 gear set (which allows you to choose from a 3.90, 4.10, or 4.11, 4.33, 4.56, 4.88, 5.13 or a 5.14, 5.38, 5.57, 5.86 or a 5.88, 6.13 or a 6.14 with the close ratios differing by a few hundredths due to different tooth counts).
There are aftermarket gears that allow you to mount a 4.10 on a three series carrier or a 3.73 gear set on a four series carrier. There are also shim rings and long bolts to mount a three series gear on a two series carrier but I do not recommend them as they almost always fail in use.
A PosiTraction from Chevrolet will be an Eaton and use clutch discs that can be rebuilt. If the rear end where out from under a Pontiac Grand Prix or an Olds 88 then the limited slip rear end would be from Auburn Gear and will use cones that can not be rebuilt. The other GM vehicles that also used a 12 bolt rear end had different gear ratios than the Chevy such as a 3.23 (for example the 3.90 was Pontiac's answer to Chevy's 4.10 as almost every GTO had a 3.90 if performance oriented; very few if any Chevy's have a 3.90 gear set).
Since there were a whole bunch of non Chevy vehicles that were built with a 12 bolt rear end and very, very few 427 SS full size Chevys ever built I would suspect the guy selling this rear end is doing some creative marketing. I would not buy the rear without first pulling the inspection cover and looking inside. It will tell you the gear ratio (divide the pinion gear into the ring gear) and if and what type limited slip rear end you have. You also need to count the number of bolts on carrier as Olds was infamous for building ten bolt rear ends while using 12 bolt inspection covers.