There are many types of classic differential; the basic function of a classic diff is no different to any modern differential it’s just the technology used in producing a modern day diff that is totally different.
In fact you can find all sorts of classic differential just like you can a modern day one: standard open diff, 1 way / 2 way / 3 way LSD’s and welded. This makes finding the correct classic differently oil vital in protecting the components and ensuring the maximum performance and efficiency is achieved.