The rear 'wedge' or reverse draft isn't really a regional thing and it certainly was not used on all Lehigh area pieces. You can find it on rifles of various areas and on European work as well. On some it's a little spike, on some if the plate doesn't come to a point, it's a wedge etc. As Dane noted, there's really no need for it as if the plate is inlet well, the tang bolt alone should hold it from moving and also if you have the guard bow return touching the plate, that also will hold it in place.
If I want to spike the rear of the trigger plate, the way I do it is possibly different than what others do so I can only speak for myself. I inlet the plate almost complete, then just hammer out the tail a little and reshape to the reversed draft or spiked point, then drive that back into the wood and finish the final plate inletting, very easy since the majority of the inletting is already done.