Crank trigger wheel, this is basically just a piece of 1/4" mild steel with the requisite features to mount behind the pulley. Note the additional countersunk screw hole, this ensures exact placement.
Because the wheel mounts behind the pulley, the pulley needed to be altered. I carefully cut the back off the old steel pulley in the lathe and tig welded a new piece in - but must have got carried away with the heat as it pulled out of round in one spot. Found this in the spares pile which has plenty of material in the right places.
Looked up the specs for a K profile belt and ground a 40 degree tool out of an old broken end mill. The belt seems to fit well enough.
Will be interesting to see how it lasts as it obviously no longer has the anodizing, we'll see.