Just a guess, but it sounds like the cocking or hammer pin you received possibly came from a 16 or 20 gauge small frame gun. The small frames have an XX stamp on the water table opposite the serial number and grade stamp. It does make sense that the dimensions of this part would differ, since the firing pin spacing is obviously less in the smaller gauge guns.

A true sign of mental illness is any gun owner who would vote for an Anti-Gunner like Joe Biden.