Band's a Glass act.

Conventional wisdom says that the era of artist development is over in the music industry, and the slow-rolling hit is a thing of the past.

Don't tell that to Daniel Glass, president of the New York independent label Glassnote Music, whose English folk-rock act Mumford & Sons has invaded the top 20 of the U.S. album chart after more than six months on the market.

Glass said, "In our initial meetings with the band and management, we used words like 'gradual,' 'longterm,' 'credible,' 'integrity,' 'no hype,' 'let's wait.'"

The London group's debut, "Sigh No More," has sold 191,000 copies here in its 24 weeks on the chart, according to Nielsen SoundScan …