The distributors on the B18 and B20 both rotate anti clockwise but the points are different. As you say, they are "reversed". i.e. On the B18 as it rotates, the cam passes the points before it gets to the pivot post . On the B20 it is the other way round with the cam passing the pivot post before it gets to the points. Both dizzys will work on either engine but there will be small differences with the advance curves and they could be either vac or no vac types. I'd guess that the B18 dizzy was fitted to the B20 when it was swapped. If so, you just need to buy the B18 type. Take your points in with you or a good photo when buying.