Does anyone know when the DSC took over the allocation of all shipments of S&W revolvers to non-military users? Pate's book vaguely mentions September 1941.

Later than that. I don‘t have an exact date, but we have direct shipped guns to other destinations into Jan. 1942.

It may coincide with the executive order freezing all civilian sales from around that time.

Did you look at the chronology in Appendix D (P. 317)? It outlines the steps, with first communications indeed in September. But there is nothing definite about when exactly the DSC had control over ALL non-military production.