Roshi, Piettas have a more trumpet bell shaped bottom on their grips than found on the Colt/Uberti grips. The waist is also thinner on the Pietta. Here are the dimensions and picture from one of the grip makers sites.
Uberti/Colt dimensions:
•Front vertical edge (A) = 1.06 inch
•Overall height (B) = 3.125 inch
•Width of base (C) 2.187 inch
•Midpoint of A to rear edge (D) = 2 inches
Pietta Dimensions:
•Front verticle edge (A) = 0.95 inches
•Overall height (B) = 3.2 inches
•Width of the base (C) = 2.4 inches
•Centerline of front edge to rear edge (D) = 1.7 inches