There are several versions of the TN55. A, S, D, V or F (vineyard), cab, no cab, 8x8 or 12x12 (at least in the larger models of the TN series), maybe even a 16x16, 2 shuttle shifts (power reverser or not), 2wd or 4wd.
S=Supersteer. Includes 4 wheel brakes, and has other premium features (such as adjustable 3 point end links).
I think the D or A model is a more stripped version. Not all premium features IMO are worth it, but most are sure nice (such as the collarless quick connect hydraulics -- not worth the price to me on repair).
Early models have square headlights, newer models have shaped headlights and a dropped hood. Early models sometimes have problems with hood paint (I think the upward exhaust models have the exhaust too close to the hood, not sure). Also, factory loader frame on the early models can crack and the mounting bolts need to be checked (bolts can break or hog out the holes). The loader remotes for the factory hydraulics are only on the rear for the early models. The newer units offered mid-mount remotes, making the hoses route much better and leaving more room in the rear for remotes.
Models without cabs may have electrical problems with corrosion if stored outside.
The biggest difference in the TN55 versus the other model TN series is it has a lighter front axle (4wd models). Search around and you can confirm the details of what I wrote (or correct them).