Inside Report (1)

The investigation of corruption charges against Reagan's labor secretary appears to be in its final lap.

Special prosecutor Leon Silverman, a prominent New York City attorney, was named almost four months ago to investigate allegations of corruption against Labor Secretary Raymond J. Donovan. Silverman says he and his staff will reach their final conclusions ''sooner rather than later'' and indicated that the finding could be released in May or shortly thereafter.

Silverman was appointed Dec. 29 by a panel of three federal judges in Washington to probe charges that Donovan lied to a Senate confirmation committee and violated other federal statutes.

Knowledgeable sources close to Donovan agree that the investigation is in its final month. They say Donovan is confident he will be cleared of all charges. But some remain perplexed why the process has taken so long.