Jason Greene is correct that Southern never had car classes. Any photo you see of a Southern freight car with a class designation was taken after the Sou/NS merger. At that time, the new NS Mechanical Dept (senior people were mostly ex N&W) simply expanded the N&W class system by adding an “S”. So BSnn indicates a Southern box car. N&W classes were not modified as a result of the merger. (I have never noticed if the same class number appeared on matching Bnn and BSnn cars. As the N&W built a very high proportion of its hopper cars and the Southern did not, I don’t expect there were many exact matches.)
Caution….many cars with complete Southern paint were given the new class stencils (I have been presented with photos as “proof” of Southern car classes. That may appear to have been from when the car was built but that is not correct.
PS After all of this…Southern DID have car classes for a very few years after the railroad was formed. There is an artifact of that on drawings into the 1900s. Usually in the upper right corner you will see a “Class” block. After classes were dropped, “Class” became something such as “box car”. (As they say..”never say never”.)