Gene,  With my 'walls with SST, the ignition of the primer becomes a little spotty without benefit of fly, as the sear will sometimes drag slightly across the half cock notch as the hammer swings down.  This is because the movement of the sear is not quite so positive from the strike of the knockoff when firing from set position as from the push of the top of the knockoff while it is still being acted upon by the unset trigger.   

This situation was dealt with by Winchester with the fly in the hammer, a groove in the sear to clear the fly, and a reduced strength sear spring to allow it to retract more easily and drop more slowly than normal for the standard trigger.  Collectively and when adjusted properly, this results in a fairly consistent performance by the SST, though not as good as either version of the DST.

HTH, Froggie

AFAIK, the slot and special fly to lower the hammer to half cock were a standard feature of all of the 3rd Model Winders if not of all Winders.  FWIW, when I put a SST on my converted Winder-cum-sporter, the original fly seemed to allow the SST system to work too.  When I tried an SST without the fly, dragging on the sear and misfires occurred with regularity.  JME, YMMV.   8)

Froggie

All the coil spring hammers that I've seen personally have been cut for the fly, but that's a mighty small sample.
FWIW, Joe