series 70 hammers on the half cock notch had a HOOK on the end.
series 80 hammers on the half cock notch had NO Hook on the end.
so on a 80 series gun if you cock the hammer back slighty to the half cock notch, you can RESET the hammer forward by pulling the trigger, the series 70 hammers you can not do this.
most likely the hook on the 70 series hammers was ground off in favor of the 80 series style, because to reset the hammer forward you hand to fully cock the hammer and then thumb it down while squeezing the trigger, which is an UN-SAFE thing to do on a loaded gun.
by pulling the trigger the on an 80 series the hammer would "click" forward, without enough force or momentum to strike the firing pin and fire the gun.
if you dont like the 80 series style then you should be able to put in a 70 series hammer without any problems, and (to my knowledge) it shouldnt affect the other 80 series FP safety mechanism.