Consider the critical Galton-Watson branching system with infinite variance of the offspring law. We provide an alternative arguments against what Slack~{\cite{Slack68}} did when it seeked for a local expression in the neighborhood of point $1$ of the generating function for invariant measures of the branching system. So, we obtain the global expression for all $s\in[0,1)$ of this generating function. A fundamentally improved version of the differential analogue of the Basic Lemma of the theory of critical branching systems is established. This assertion plays a key role in the formulation of the local limit theorem with explicit terms in the asymptotic expansion of local probabilities. We also determine the decay rate of the remainder term in this expansion.