Nearly mate!

T2 (which is also T25 and T25 IIRC), T3, T4, T6 frame is purely the flange size on the manifold to turbine housing!

T2 is smallest, T6 is massive.

Cossies run T3 size ones for example (T4s and T35s just have T4 sized compressors with T3 sized housings).

IIRC a 66-1 is just one of many different 66mm inducer compressor wheels i think, a Turbonetics name.

As some compressors, even tho they same size, are more efficient than others, better at certain boosts, blah blah.