My service bulletin indicates the use of 1 1/2 pints of M-533. in the ****** and differential, and on another page, simply states "gear lube" in the differential. The "Gear Lube" could be misleading as Rear Ends are different today, the seals are different today. Certainly "Gear Lube" in 1931 is different than "Gear Lube" today.
I am not sure if 600 W was ever officially recommended? It is used a lot. I has been around for a very long time, it pre-dates the Model A. I do not know if it contains the EP additives required for a differential or not. Snyders interchanges the names 600-W and M-533, but again, I have never seen any documentation that they should be interchanged. I would certainly not mix the two if I some 600 W. I am sure someone here knows, but I have never seen the data.
I bought the M-533 for my current build. Because I do not know what has been put in before I am draining and re-filling both.