‘ And he wrote there on the stones a copy of the law of Moses, which he wrote in the presence of the children of Israel.'
These need not have been the same as the altar stones. The Hebrew definite article is not specific. It can simply mean ‘on the stones I am now talking about'. The stones would be plastered white (an Egyptian method) and then written on in a kind of primitive ink. The copy of the Law of Moses probably refers to the covenant containing the ten commandments of Exodus 20:1-17. It may, however have included parts of Deuteronomy.