Pettifogger is referring to size and shape. The 51 and 61 are the same except for the material, the 61 backstrap and triggerguard are steel, the 51 is of brass, except the London Model which is/was steel. Original Colts 51 brass gripframes were Silver plated, but it is worn off of most originals except those kept in pristine collector's condition. Silver can be detected in the corners of many originals.
The 1860 is longer by about 1/4 inch, and has a brass triggerguard and steel backstrap, except for the "Civilian" model which has a brass backstrap as well.
Original Colts had the Navy size gripframe on the SAA as well, but it seems the Italian repros SAA gripframes look a bit more beefy than the 51/61 frames, perhaps it's illusion. I haven't tried to swap the wood between the Italian SAA and Navies.
All the metal parts of the gripframes are interchangable as far as the screw pattern between the 51, 60, 61 and SAA, but as the finished receiver frames vary in actual dimension, some are wider or narrower, or assymetrical when finally fitted to a particular frame, so swapping can be a problem between finished guns.