The threaded firing pins were used in the very early years and probably changed at the 90,000 SN level.
The one in your picture sure looks pinned on to me.
The shape of the various parts has transformed over the years but functionally they remain the same and are interchangeable with some fitting.
The 99 action is pretty stiff by nature. Be careful not to soften it up too much or you will start to get lever droop when it's fired.
