A12 GX - early block with distributor up front, and has provision in the lower water inlet for the plumbing for the water outlet from the inlet manifold. Round dome top hitachis (SU's). twin outlet cast iron hearders to twin pipe engine pipe.

A12T - late block with dizzy in the middle and square top hitachis. + the water plumbing as mentioned above.

These features are what I look for when looking at these type of engine designation.