NOWADAYS, IF A musical show lasts more than 20 minutes on Broadway, there is an original-cast album - and sometimes, one suspects, the making of the album has been one of the main reasons for producing the show. This is a comparatively recent development; nobody thought of doing a complete original-cast recording until Oklahoma! came along in 1943 and revolutionized the business. But, as a result, we have lost a lot of basic documentation on the performance history of America's most distinctive contribution to musical theatre.

It is not that we completely lack recordings of pre-Oklahama! musicals; when records became a profitable enterprise, the stars of popular hits began recording some of their most successful numbers, but most of these old singles have long since dropped out of circulation and out of memory.

Now, with a fine blend of historic piety and show-biz know-how, the Smithsonian (that liveliest of museum organizations) has begun to produce recordings in the "American Musical Theater Series," ferreting out and transferring to long-playing discs the bits and pieces that remain from some of the musical comedies of the remote past. The first three records in the series, The Ziegfeld Follies of 1919, George and Ira Gershwin's Lady Be Good and Cole Porter's Anything Goes, do not quite constitute original-cast albums, but they do preserve performances of this music by people who were in the show when it was on Broadway. The missing pieces can never be recovered, but what remains (aided by very full annotations) gives a good idea of what these shows must have been like when they were new.

The three shows offer a variety of attractions. In the Ziegfeld Follies of 1919, most of the material is topical, with novelty numbers about the end of World War I and the beginning of prohibition by various songwriters, of whom only Irving Berlin is still known today.

Berlin is represented by the novelties: "I've Got My Captain Working For Me Now" and "You'd Be Surprised," both worth knowing and distinctively performed by Eddie Cantor, and by the better known "A Pretty Girl Is Like a Melody" sung in the period style by tenor John Steel. But the record is dominated by Bert Williams, a light-skinned black entertainer who had to put on blackface for the novelty numbers he sang. A half-dozen of these are on this record (it is not completely clear how many, or which ones were actually sung in this show) and he performs his often stereotyped material with a subtlety of style, a dramatic flair and a sense of timing that make everyone else in the show sound slightly amateurish. If there is enough material in the archives, a whole record devoted exclusively to his art would be well worth producing.

In Lady Be Good, besides the title number (which is given in two very different performances), there are three versions of the show's other big hit," Fascinating Rhythm," including two with Gershwin at the keyboard. Gershwin also plays a piano transcription of "The Man I Love," which was dropped from the show early in its run, and there are several vocals by Fred and Adele Astaire as well as some weird novelty singing by Clif Edwards (Ukelele Ike).

Not to be outdone by pianist Gershwin, singer Cole Porter appears in Anything Goes, performing with a voice that can best be described as not bad and revealing an excellent sence of how to handle his own intricate lyrics. Of equal interest is an early appearance of a young Ethel Merman who sounds very much like the Ethel Merman of later decades. The record is filled out with versions by members of the London cast, who seem somewhat less at home with this American material but supply interesting contrasts with the original production.

The first three offerings in the "American Musical Theater Series" are so good, not only in terms of historic interest but in entertainment value, that I look forward to later issues. Of the three, I find the earliest, Ziegfld Follies of 1919, most fascinating, but lovers of the American musical will want to have them all.