I'm not an expert on the matter, but as far as I can tell the first versions:
'Squared' trigger guard interior, older Ambi manual safety with low shelf, grips with tapered-in top sections, older non-paddle extended magazine release ('Ace Frehley Boot')
Later/current versions:
'Rounded' trigger guard interior more like the regular SP01's and 75B's, newer 'Shadow 2 style' thumb safety with higher shelf (flat on other side), grips with slightly different cut (again more like the Shadow 2's), magazine release with orange pad adjustable for angle, different profile to slide lock lever.
I don't know when they switched to aluminum triggers. Also, I think there may have been some 'transitional' models in-between that had some of the feature changes like the trigger guard before the others. But these are the most apparent ones, and I think they are all interchangeable.