I got a trigger wheel made for my bottom damper. This is one that KAD specially made for me.
The trigger wheel is actually bolted to the back of the pulley via a small Z adapter and the centre is machined out so that it is solid all the way through to the cam pulley. This is to ensure that crank vibration doesnt make the pulley drop off.
The trigger wheel is bolted to the back so that it avoids timing scatter that can occur with the metalastic damper. It has been known for the outer ring of the damper to drop off when they start to get really worn....so you can imagine there is plenty of scope for timing error on even a relatively young damper.
The Z adapter is there so that it gets around the tight fit of the timing cover.