What does z in lac z, y in lac y and a in lac a gene stand for?
The Jacob and Monod article on regulation is paywalled, but there do seem to be some PDFs floating around.
In this article, the terminology of z and y is introduced as designators for mutants (z+ and z-, y+ and y-).

The z appears to be linked to a term from a kinetic equation.
The y is not justified, implying that it is an arbitrarily chosen variable, and such choices are often by convention adjacent to the justified variable (e.g., the index variable i is often followed by additional indices j and then k).

The a component is not present in the original article, but its name seems likely to come as an abbreviation of its function as an acetyltransferase