WASHINGTON — It was the first public allegation that CBS News used forged memos in its report questioning President Bush's National Guard service — a highly technical explanation posted within hours of airtime citing proportional spacing and font styles. But it did not come from an expert in typography or typewriter history as some first thought. Instead, it was the work of Harry W. MacDougald, an Atlanta lawyer with strong ties to conservative Republican causes who helped draft the petition urging the Arkansas Supreme Court to disbar President Clinton after the Monica Lewinsky scandal, the Times has found.