Look and alignment are indeed different in the 5 / 7 / 9 / 11 models, with the 5 and 7 being closer to blade performance side and 9 and 11 being true mallets. Some people prefer different head shapes and alignment lines. As for the even and half numbers, those are differentiated by the hosel designs on each. The 5, 7, 9, and 11 will be nearly face-balanced for a straight back, straight through stroke, and the 5.5, 7.5, 9.5 and 11.5 will have slightly more toe hang for a more arcing stroke.