...they were all made with the vertical stroke first. If we look at them again
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it can be seen that the difference between them is that in some cases the vertical is deeper than the horizontal, in others, the reverse is true. But apart from that I can't see any difference between them as far as order of wedges is concerned.
I think what happens when a wedge is made (or, to be precise, what seems to have happened when the above wedges were made) is that the clay is compressed rather than moved out of the way. The compression leaves no trace that I can see; since there is no movement, primacy is not detectable.
So it seems that at least in the case of the wedges made in this little experiment, John's memorable dictum, the clay has to go somewhere, is not true: it is simply imperceptibly compressed.
I would be very happy to be proved wrong in this...