(MONEY Magazine) – -- Following MONEY's March story ''The Pros Flub Our Third Annual Tax-Return Test,'' many readers have requested a copy of the tax preparers' quiz. It is available by sending a $5 check or money order to the MONEY Letters Department at the address listed below. -- A number of readers have asked for the name of an organization that can provide information about the value of their coin collections. Write or call the American Numismatic Association (818 N. Cascade Ave., Colorado Springs, Colo. 89803; 800-367-9723).