"Style." Manhattan Transfer's Allan Paul breaks into a huge grin and his eyes have an I-was-ready-for-that-question-wasn't-I-look. "That's what makes us visible, keeps us fresh and growing. And that's what the audience likes about us."

Paul and group vocal arranger Janis Siegel sit in the dressing room at Constitution Hall after Manhattan Transfer's recent appearance here and talk about why two people in their mid-20s are singing songs originally written as far as the 1930s.

"I don't even think of music in terms of time." Siegel seems almost offended. "When you go to a classical concert and hear Bach, you don't sit there and say, 'Oh - this is old music.' It's music, period. We do songs becuase we like them and we give people our own interpretation, our own personalities."

Manhattan Transfer also gives people ballads, swing tunes, rhythm and blues, harmonic triptychs, greaser love songs, shimmy-shimmy-ko-ko-bop and other styles that have been termed everything from superb to ersatz. Assorted modes beg assorted comments, but it's not hard to understand why the group has garnered a reputation for slickly packaged camp; a kind of Las Vegas Sha-Na-Na image that it considers unjustified.

Their visual approach is essentially a throwback to the old days, with gowns and shades, cowboy suits and sequined blouses. Each part of the act comes from another era, and even extended bits (the '50s greaser usually is good for three of four numbers) tend toward the ephemeral. As for musical focus - well, there is no focus.

Their new album, "Pastiche" (Atlantic, SD-19163) is just what the title says. One side includes Cole Porter ("Love for Sale") and Duke Ellington ("In a Mellow Tone," dedicated by Siegel to Ella Fitzgerald) while the other features contemporary composers like Rupert Holmes, Gerry Goffin (of Goffin and Carole King), and Motown's Holland/Dozier hit machine. Consistent it's not.

"I think it's unfair to peg an act and say "They do that,'" Paul says with a hint of annoyance. "People called us nostalgic, though that's just about finished now. How could we nostalgic? We weren't even born when a lot of this music was popular."

Good point, and it singles out another contradiction in Manhattan Transfer's makeup. Though most of the quartet and its supporting band are under 30, its music is astoundingly devoid of rock'n'roll. "Not so," responds Paul with Siegel nodding in agreement. "We do a 2 1/2-hour stage show that has a kot of rock in it. We do the '50s songs and some '60s songs." Paul suddenly looks a bit defensive. "We like rock'n'roll, we're aware of it, and it's part of our act."

Not so. A few "American Graffiti" tunes is not Bruce Springsteen, and the group's 60s remake on "Pastiche," the Supremes' "Where Did Our Love Go," is the album's weakest track. The group seems to have no sense of the song's depth (it is sung here in near falsetto) and Gene Page's arrangement exemplifies the group's frequent tendency to substitute technique for emotion.

Whe the technique and emotion mesh, though, the ensemble is as good as, any in show business. "Four Brothers," originally composed for Woody Herman's band by Jimmy Giuffre (with the lyrics added later by Jon Hendricks), is a vocal masterpiece.

Arranged by Siegel, the piece swings forcefully but the voices control the tempo and dynamics, and the piece has more things happening in 3:47 than Phil Spector would have though possible. Lyric interplay - the Manhattan Transfer trademark - blends so seamlessly that you have to listen very closely to make sure that it is four people singing and not a studio overdub.

The rest of the album is predictably unpredicable. The standards are adequate Xeroxes of the originals and "Walk in Love" is a pretty pop tune with warm harmonies. "It's Not the Spotlight" comes off harsh, but it also comes off harsh by Kim Carnes and others who perform it, so it may just be the song. This album's urprise (there's usually one in each album) is "Je Voulais (Te Dire Que Je T'Attends)" by Michel Jonasz, sung entirely in French. Now the band's not only timeless, it's international.

"Pastiche" reaffirms a widely held notion that Manhattan Transfer is technically fluid but personally indistinct. Janis Siegel, naturally, resents the accusation.

"Songs in the '30s had a distinct sound. Jazz in the '40s and '50s had a distinct sound. Even the music of the '60s sounded a certain way. The '70s doesn't have that. What we try to do is present the music as best we can and be as faithful to the original as possible."