"It was the most avant-garde band of its time. Thirty years later the scores have not dated, and we'll prove it to a couple of hundred people on Monday night."

Jack McKinney's enthusiasm is understandable-for 10 years he has been preparing a biography of the late Boyd Raeburn, and tomorrow night will bring the first presentation of the music of the Boyd Raeburn orchestra in three decades. The virtually forgotten scores came to light only recently, and will be performed by Mike Crotty's Sunday Morning Jazz Band, an 18-piece ensemble of local musicians, at Frankie Condon's Supper Club in the Rockville Plaza Motel in Rockville. Several former members of the Raeburn band will sit in with the Crotty group.

The Boyd Raeburn Orchestra is remembered by those who heard it in the 1940s as a band with a very new sound-it was, in fact, ahead of its time. Its harmonic concepts were advanced, it made use of fluctuating time signatures (4/4, 7/4, 11/9 in one piece), it resembled a symphonic group on stage with flutes, oboes, bassoons, alto and bass clarinets. English horns and harp supplemented its standard swing band instrumentation, and it sometimes employed classical themes. Buddy DeFranco, with the band in the late '40s, recalls the arrangements as the most difficult music he has ever played.

McKinney, a professor of English at Bergen Community College in New Jersey, is careful to point out that along with the intellectual design, creative experimentation and innovative urge that characterized the band there was the highly emotional music that it generated. It had, in a word, swing.

Many musicians of merit, and some of international fame, passed through the Raeburn Orchestra during its peak years of 1943-1948. Among those were DeFranco, Dizzy Gillespie, Tommy Young, Don Lamond, Oscar Pettiford, Maynard Ferguson, Benny Harris, Hal McKussick, Al Cohn, Serge Chaloff, Frankie Sokolow, Johnny Bothwell, Dodo Marmarosa-and Al Swope, Steve Jordan and Angelo Tompros, three from the Washington area. Featured vocalists of the band included Ginnie Powell, who became Raeburn's wife.

Boyd Raeburn's associations through the '30s had been largely with hotel dance bands of little distinction, some of which he led. Toward the end of the decade his band moved in the direction that the major swing orchestras of the day were taking. Circumstances, both cultural and personal, conspired to transform him into the director of an organization, the Boyd Raeburn Orchestra, that was on the leading edge of advanced musical expression of its time.

A new body of musical thought was coming to the fore in the early '40s; for want of a better term we now call it "modern jazz." The big dance/swing bands still dominated the scene, both popular and jazz, but several of these were listening with interest to the new music coming to be known as behop. Woody Herman changed the image, and the sound, of his orchestra from "The Band That Plays the Blues" to the first of the "Herds." Stan Kenton incorporated bop into his offerings, styling his new approach "Progressive Jazz."

No one claims that Boyd Raeburn was a creative artist of great stature; he himself was heard to remark, "For a musical idiot I've got a great band. But few who are familiar with the music of his period will deny that his band ranked with the best of the innovative groups of the era, including those of Kenton and Herman.

But Raeburn was creative in one sense: He was adept at recognizing talent and he both knew what he liked and wanted to improve his band. A crucial change of direction for his purposes occurred here in Washington when a number of local musicians were hired for a performance at the Roosevelt Hotel in January 1944. As was often the case on such "gigs, the performance was broadcast on the radio. Among those who joined the band on this occassion was the new arranger Eddie Finckel, whose harmonic innovations and orchestral design would dominate the scores for the rest of that year.

When the band opened a month later for an extended engagement at New York's Lincoln Hotel, its reputation had arrived already and people thronged to attend its first performance. Roy Eldridge, hearing a few numbers on his bedside radio, got dressed and hurried down to sit in with them. Raeburn put him on the payroll for the rest of their month-long stay at that location. It was at this time also that Dizzy Gillespie rewrote his "Night in Tunisia" for The Raeburn Orchestra's use.

Gene Krupa hired Finckel as 1945 approached, and Raeburn began looking around for a new arranger. The search was simply for a competent writer, but what he found was an extraordinary talent who would turn out to be not only a very inventive composer but somewhat of a musicial seer as well. George Handy joined the Raeburn organization in early 1945 and it was his influence that held sway for that year and the next.

A few months after Handy was taken on board the band found itself in residence at the Palace Hotel in San Francisco. Handy had been searching for his identity and, as he tells it, "One night I sat down and my vision seemed clearer-the fences were down. I suddenly seemed to understand what I was doing." He wrote "Out of this World" (biographer McKinney says it was exactly that), the composition that is regarded as the first modern number of the band. It was impressionistic, rhythmically complex, harmonically sophisticated and it had that indefinble, but essential, essence of swing, without which a performance is not jazz.

The instrumentation of the band had been gradually augmented with additions from the reed, woodwind, brass, string and percussion families and this continued into 1974 under the direction of a new arranger, Johnny Richards, who had replaced the departer Handy.

Richards, whose approach was that of a romanticist, left his stamp also on the Boyd Raeburn Orchestra, contributing 50 compositions during his year's stay. A multi-instrumentalist whose mother was a French concert pianist, Johnny Richards went on to arrange for Stan Kenton, Dizzy Gillespie and others. He died in 1968.

Boyd Raeburn, part visionary and part organizer, fashioned a musical entity which could perform and extend the current ideas in a big-band format and he achieved this by seeking out the people who could write and play those concepts. As the '40s came to a close his organization began to wind down.

By the early '50s, Boyd Raeburn was no longer traveling with a band and had settled in New York. He occassionally took pick-up groups into supper clubs in New Jersey. It was a long way from the days when his orchestra had moved from one coast to the other, when the sidemen had included the likes of Dizzy and Trummy and Johnny Bothwell, when Duke Ellington had given both moral and financial support to the band. In 1954 Raeburn became a partner in a furniture business; several years later he moved to the Bahamas where his wife, Ginnie Powell, died in 1959. Boyd Raeburn's last years were spent in obscurity; he died in Indiana in 1966.

That Mike Crotty's Sunday Morning Jazz Band will for several hours tomorrow night recreate some of the Boyd Raeburn book is something of an anomaly. The Sunday Morning group is as intent on the exploration of the current and future sounds of jazz as the Raeburn group was. They consider themselves a jazz orchestra, not a repertory group.

The project came about as part of a nationally syndicated radio series last year which was terminated before the airing of the already prepared Raeburn material. Bill Schremp, baritone saxophonist of Crotty's SMJB and researcher for that aborted project, says that some 60 pieces of music had been readied for the radio show and that "it just took on a life of its own." Schremp's work involved not only the physical restoration of the manuscripts ("I feel like a conservator cleaning the Mona Lisa.") but the writing of missing parts. He fondly likens the trunkful of scores to "a box of antique car parts out of which you're supposed to make a good Model-T."

So tomorrow night Mike Crotty's Sunday Morning Jazz Band will become a replica of Boyd Raeburn's Surrealistic Orchestra, which is how NYC's Zanzibar billed them in January 1947.

Some former members of the band, arranger Eddie Finckel and its historian/biographer/discographer Jack McKinney will be honored guests. Toni Wilson will re-create the vocals of Ginnie Powell, Bob Drummond, those of David Allyn, and the sounds of the evening will have names like "Dalvatore Sally," Boyd's Nest," "Tonsilectomy," and "Two Spoos in an Igloo." CAPTION: Picture, Boyd Raeburn (left) and his orchestra. Its instrumentation included flutes, oboe, bassoon, alto and bass clarinets.