We need to look at the style of the letters X and R, and additionally bust style to differentiate between the 5d sub-classes.
5a1 : X2 R2
5b1: X4 R1
5d1: X5 R3 - Crude bust Note that 5d coins have a characteristically different bust appearance, and unusual reverse spellings.
5d2: X5 R3 - More realistic bust
5d3: X5 R3 - Neater bust, and distinctive sharp crown ends rather than half fleurs.
Some examples of X/R lettering: