Forty-Second Street -- At the Kennedy Center's Opera House through July 27.

The 442d show in recent memory that takes a nostalgic look at the good old days of Broadway, when original shows were being done on Broadway instead of only nostalgic looks at the good old days of, etc., is 'Forty-Second Street," having a pre-Broadway engagement at the Kennedy Center for the next month.

Recent memory may be somewhat off: R.M. has been so sung-and-danced at by '70s and '80s revivals of '20s through '50s musicals, unending runs of '60s musicals about the '50s, and '80s musicals about the '40s and the '70s that it no longer knows when it's at.

"Forty-Second Street" calls itself "a 1980s musical about the 1930s." It consists of song-and-dance numbers from eight movie musicals made from 1933 to 1937, strung together by a plot that's older than God -- the one about the chorus girl who gets her big chance when the star breaks a leg at the last minute.

It is, in other words, a faked antique. But it's an amazingly well-crafted one, on which money and effort have not been spared.

Robin Wagner's scenery and Theoni V. Aldredge's costumes are sensational. Gower Champion's choreography is masterful. Producer David Merrick, whose 84th show this is, has been wildly lavish.

"Shuffle Off to Buffalo" (from the 1933 film of a "Forty-Second Street") is done on a train that moves. "Keep Young and Beautiful" ("Roman Scandals," 1933) has a two-tiered, mirrored, revolving gymnasium and an entirely different evening costume, complete with hat, for each chorus girl. aA full company, for "Lullaby of Broadway" ("Gold Diggers of 1935:), means that the stage is really filled and that the dancers' tapping shoes are as loud as the orchestra.

Even with all those shows about Broadway's spectacular past, it's rare to see such spectacles on the stage now.

There is, of course, some irony in the fact that the movies had to supply this material for celebrating Broadway. But the chief trouble, for devoted Broadwayites, will be one that's inevitable in a fake, for those who have the eye to see it.

Satire can look frankly from the present to the past, and revivals, if faithful, continue the past into the present. But a pastiche such as this, which uses genuine materials from the past to make a new item, like a Louis XV chair newly made out of worm-eaten old wood (as opposed to a crude copy such as last year's '40s fake, "Swing," which was made out of cheap materials), shows the heavy hand of the copier.

It's not just the later influences, such as the way the opening number, "Audition," seems to strain to refute, with determined, cheer, the psychological heaviness of the similar opening of "A Chosen Line," or the finale's debt to "Slaughter on Tenth Avenue." It's the slight air of superiority with which the no-longer-accepted conventions of the past are treated.

Sentimentality was a feature of '30s shows -- but not because people didn't grasp emotional complexity in those days, any more than the car chase is a feature of modern films because people nowadays don't understand traffic laws. To treat the old convention as if it represented old reality is itself naive, as well as patronizing. When Jerry Orbach speaks in Tough Talk ("Now listen -- and listen hard") and Wanda Richert speaks in Sweet Talk ("Gee, that'd be swell") and Don Crabtree talks in Country Talk ("I never seen anything so purty"), they make it sound as if it comes from the Dumb Old Days, rather than good, as in good-natured, ones.

Tammy Grimes, however, does an honest job of playing the tough veteran star. It's unfortunate that Richert, working away at the chorus-girl role, never exhibits her kind of star quality, which sort of negates the premise of the show.

The staging is rich enough to make "Forty-Second Street" popular with new audiences. But it would be a gyp if they took it as either an interpretation of the past or a recreation of it.