The Post's suppositions as to the alleged effectiveness of a proposed national seven-day waiting period and police background check on the purchase of handguns {"The Brady Bill: Decision Time," editorial, June 26} are refuted by the 24-year history of a similar law in Maryland.

Since 1966 the concept behind the Brady Bill -- denying the retail availability of handguns to criminals by a waiting period and background check, has been a fact of life in Maryland and has proven to be just another fallacious ''gun control.'' This law is not restricting the availability of handguns to career criminals as evidenced by the steady increase in Maryland crime rates since 1966. And there is no record of any criminal having been detected, prosecuted and convicted as a result of this law.

On an average of 25,000 handgun purchases a year, less than 5 percent are disapproved as a result of the state police background check. Of the disapprovals appealed, 50 to 85 percent are subsequently reversed due to erroneous criminal justice records.

A report by the Maryland Criminal Justice Coordinating Council states: ''In particular, the policy implications of further gun controls ... are such that one becomes resigned to the fact that guns cannot be effectively regulated out of the hands of criminals.''

What Maryland's law has achieved is the creation of a repugnant system that denies citizens the legal presumption of innocence, makes them the object of police investigation by virtue of their exercise of the constitutional right to keep and bear arms and permits them to be victimized by an inept criminal justice records system.

Maryland should serve as the paramount example of why the Brady Bill should be defeated. W. VERNON GRAY III California, Md.