https://www.defender.com/product3.jsp?path=-1|311|2349117|2349129&id=685485

My J/30 is a 1984 model with a 1" shaft - so 1/4" is the size for me.
I do not know if that would be the same with earlier modesl 3/4" shaft.
I love the GTU, have been using it for years now: it does not overheat even if you get a bit paranoid on the tightening, it does not bite (too much) into the shaft either.