Actually, the whole trigger group assembly is the same for the Security Six, Police Service Six and the Speed Six and it doesn't matter if it is a low back (s/n 150-) frame or a high back (151- and up) or Speed Six frame, blued or stainless steel.
Also, the hammer, hammer dog, hammer pivot pin, hammer spring, hammer strut, cylinder assembly, and barrel (in its different configurations) will fit in all models of the Six series revolvers.
What is different is the shape of the grip portion of the frame between the low and high backs (SS, PSS) and the grip portion of the Speed Six frame.
Oh, also what is different is the size/shape of the front sight between different models/barrel lengths. There are models like what were built for the PA State Police that had a front sight designed specifically for the ammo they used.