Personally I find the differentiation of these coins a bit forced. The switch from small head to big head happened in 1980, although you still had small heads up to 1982. But it is one of those coins where you find a lot of variants anyway. And that is how I see it: just a coin with a lot of possible variants, most of them only visible in comparison and under magnification. In Belgium catalogues you see Morin separating them according to ruler (two kings), and Laurens in 4 types according to the head size and lettering (fine, large). Take your pick ;-)