Center Street in downtown Manassas is busy, and the noise of traffic sometimes downs out the wistful Bee Gees' tunes playing on the radio inside the Didlake Occupational Center.

The director of the center, Rexford G. Parr Jr., admits that traffic gets worse on delivery days, when trucks roll up outside the center to be loaded with rubber stamps, custom-made file folders, report card covers, preprinted envelopes and small auto part boxes. Eventually, he would like a larger buildiing and a loading dock. But, for now, he thinks the center, a sheltered workshop and training facility for mentally retarded adults, is right where it should be.

"It's fantastic," he said. "Ten years ago, you didn't see any retarded people on the streets of Manassas. They were all in institutions. Now, all the business people up and down the street know we're here and what we're doing. It's one of the really great things about being in the middle of downtown."

Didlake is a private, nonprofit foundation that receives 15 percent of its annual $132,00 budget from United Way, which is matched by funds from the state Department of Mental Health and Mental Retardation. The remaining monies come from other state and county sources and private donations. The centered open in 1965 as a school for mentally retarded children. In 1974, it became an activity center for adults, Parr said, and three years later started its vocational training programs.

The items that employes of the center produce are modest but invaluable in some ways, Parr said. "We essentially take a look at what a person can and can't do, and we try to build on those positives and help them cope with society at whatever level they're capable of." $ for some center employes, who range from mildly to severely retarded, that means operating a letter press hand-crimping envelopes for a Florida attorney's office and cutting file folders down to special sizes.

The center, whose neighbors are the Chamber of Commerce, the Elks Club and a bike shop, has a pragmatic approach to skills training for the retarded.

"The emphasis used to be on skills training only," Parr said, "with the result that a community sometimes ended up with too many people trained for one job and not enough jobs. The trend now is to develope skills applicable to a wide range of jobs."

Last year, Didlake placed six persons in outside jobs.

"That's not really high in numbers," Parr conceded, "but these people are those who were never supposed to make it."

Parr said outside employment is the goal for everyone except the most severely retarded. "We're not looking to have a large, sheltered workshop population. Anytime someone has learned enough to be productive in our environment, they've learned enough to be productive for someone else."

Parr speaks enthusiastically of one of Didlake's success stories, a 30-year-old man who lives in a group home in Prince William County and works at a tire shop in Manassas.

"Here's a guy," Parr said, "who had spent most of his life at the Lynchburg State Training School and Hospital, and if it hadn't been for the state push to get people out of institutions, he would probably still be there."

Parr would like to expand Didlake's services, which include bulk mailing and custodial work. For example, until now, Didlake employes have made rubber stamps on contract only, and Parr wants to begin marketing them independently to local, state and federal agencies, where, he says, "the potential for our product is pretty unlimited."

He also is working on a program that would place workers with local employers, who would provide on-the-job training.

Despite his plans, Parr worries about Didlake's future and the plight of handicapped persons in Manassas and surrounding Prince William County.

"This county is one of the fastest growing around," he said, "and we estimate there are probably 2,500 persons in this area with some kind of mental handicap, and probably only 75 are being served at any one time. A lot of them may not be mentally retarded but could use vocational training."

Parr said there are only two other facilities in the area that aid retarded adults. One is a sheltered workshop in Dumfries, 18 miles from Manassas. The other is an activity center in Woodbridge.

As for Didlake, Parr said: "We've gotten to the point where we're kind of bursting at the seams."

Still, Parr remains enthusiastic about Didlake's importance for mentally retarded citizens.

"Everyone," he says, "has the basic human right to get up in the morning and go to work, seek advancement, all things you and I take for granted and struggle for 30 years to achieve."