JTT,
Ain't none. Just like the .38 S&W Special/Colt Special, same case, different name.
Technically, in the really old days it was a .25-21 Marlin, always trying to one-up Winchester. The only difference was the .25-21 was very slightly longer in the neck. The Winchester cartridge can be fired in either.
If you have more than one .25-20 or .32-20 you will notice that there is a noticeable difference in the fired cases, more than I have ever seen in any other cartridge with the exception of the other Win. dual use cartridges (.38-40, .44-40). For some reason the chambers were all bottlenecked, but a lot of old black powder period ammo was straight taper llike the .32-40 or .38-55.
Now you know more than you wanted to, enjoy it, it's a good find if in good shooting condition.