Neither Shakespeare nor Shaw, Osborne nor Ayckbourn have matched Agatha Christie's box-office record. Eighteen months after her death, London has three Christie stories on its stages, with a posthumous production to make it four in September. None of the Big Guns ever has had four running concurrently.

This new one, adapted by Leslie Darbon, is from a 1950 novel, "A Murder Is Announced," rights to which Christie assigned her longtime producer, Peter Saunders. In her lifetime, Saunders had not convinced Dame Agatha that "Announced" would make a play, but to his pleasure Saunders discovered after her death that she was giving him the chance to prove it.

Saunders is the producer of "The Mousetrap," which he nursed along, after an inauspicious beginning, into its 25th year, a world stage record. While the "House Full" signs go up for each "Mousetrap" performance at the St. Martin's Theater, across the street a musical inspired by "Ten Little Indians" and which Washington and New York saw first, "Something's Afoot," has opened at the Ambassadors. The Fortune, in the shadow of giant Drury Lane, is into its second year with "Murder at the Vicarge."

Saunders beamed to learn that "Towards Zero," which he produced in 1956, was the annual Christie play at Washington's Olyney Summer Theater and added that at least three American regional theaters are programming "The Mousetrap" for next season.

Ramrod straight and ruddy, Saunders sat in his elegant oval office behind the Strand's Vaudeville Theater, one of the properties he earned through faith in "The Mousetrap." From a once wealthy family and raised in one of England's storied "public" schools, sixtyish Saunders is regarded in much the way Arthur Hopkins and John Golden were by their producing peers: a gentleman in a gambling trade. Small wonder Christie trusted him.

"The story begins," says Saunders "with the 80th birthday of Queen Mary in 1947. The BBC asked what she would most like as a salute, anything she chose. She replied she'd like - an Agatha Christie play. Agatha wrote a half-hour piece titled 'Three Blind Mice.' Later she enlarged it for the stage as 'The Mousetrap.'

"In time Agatha developed a theory about her play: 'It is not really frightening. It is not really horrible. It is not really a farce, but it has a little bit of all these things and perhaps that satisfies a lot of different people.'"

There's no question about "The Mousetrap' being a great or even exceptionally satisfying play. It isn't. And you won't find the plot outlined anywhere. No one's ever given away the ending, which might well be because a half-hour after you've left it, you'd be hard put to detail the plot.

Now 34, Mathew Prichard is the grandson to whom Agatha gave "The Mousetrap" as his ninth birthday present. "No one realized until much, much later," he has written, "what a marvelous present that was. She was an exciting person to be with because she always tried to look on the good side of things and people. She always found something to enthuse about.

"I think the play works because it has humor, drama, suspense and a jig-saw puzzle. You can take anyone from your 9-year-old daughter to your 90-year-old grandfather. There aren't many plays on the scene that have followed that recipe for success. I should imagine that the acting profession has a lot to thank my grandmother for and she in turn had a healthy respect for their talent and application."

Saunders treats his production with the tender loving care a vintage owner gives his Rolls-Royce.

The 1952 cast was headed by Richard (now Sir Richard) Attenborough and as the originals left for other roles, a tradition has grown up for a new cast every year. The eight parts now have been acted in London by 134 players.

Over the years there have been seven directors (one a woman, Joan Knight) and in 1965 the original setting by Roger Furse was replaced by a rather more elaborate one by Anthony Holland. Of the original production staff, company and stage managers, box office and wardrobe, many remained over 20 years, though by now all have retired. There have been more than 10,200 performances.

"Of the 18 Christie theater pieces," Saunders remarks, "this is one of the 11 she wrote herself. The other seven were adapted from her books by others. I've always felt that her best probably was 'Witness for the Prosecution,' which we presented the year after 'Mousetrap' opened. In the cast, however, that one has a hidden danger. It requires a star actress, but watching it in performance you can wonder why a star has so apparently unimpressive a part. That rather gives it away. We were lucky that Patricia Jessell wasn't known in New York when the play opened there. Dietrich was, of course, gorgeious in the film, but her starry presence tipped the ending."