IN the days when every eastern preparatory school for boys staged an annual Gilbert and Sullivan operetta, it was not uncommon for three prominent faculty members to appear together in a curtain raiser: Box and Cox. Messrs.  Box and Cox were two Victorian gentlemen who, unknown to each other, rented the same room from the same landlady, an economic feat made possible by the fact that Mr. Cox worked during the day while Mr. Box worked at night. The arrangement remained hidden to both tenants until Mr. Cox was let off early one day by his employer, and returned to discover his room occupied by an apparent interloper.  The farce reached a stirring climax—a scene in which the landlady, frequently played by the headmaster in a stringy red wig, at length pacified her two part-time tenants, who indeed are discovered to be long-lost brothers.