I don't know if 3/4" is 19mm, but irregardless, it's between the our standard sparkplug sizes here in Australia.
Our sprockets or whatever its called (now I'm forgetting what they're called), are 6 points.
I put the bigger one over it which goes on, but you have room to play as you rotate it. It's not a snug fit. Where it does hold on, you try loosening the sparkplug, it just slips.
So hopefully my local warehouse store will have one that fits. I'm almost doubting it though because Australia generally has nothing... Only what we use which is never the same with anyone else...