The unidentified coconspirator cited by federal prosecutors as allegedly having helped former D.C. Lottery official Alexander Exum obtain payoffs from a lottery board contractor is Hugh D. Mason III, a former city government economic development official, according to sources familiar with the investigation.

Exum, who pleaded guilty Thursday to charges related to his role in the scheme, conspired in 1984 with another person -- identified by sources as Mason -- to seek thousands of dollars in payoffs from Han Yong Cho, owner of a wholesale food distribution company, in return for Cho's receiving two lottery board contracts, according to prosecutors' statements in court.

Mason has not been charged in the Exum case and denied through his attorney yesterday that he participated in any payoff scheme. Nicholas A. Balland, Mason's attorney, said his client worked as a business consultant for Cho after he left the D.C. government.

Exum, 53, a former executive assistant and chief of marketing for the lottery board, pleaded guilty to accepting $5,500 in payoffs and the use of a new 1984 Honda, valued at $12,877, that was purchased for him by Cho.

Exum's plea is the first resulting from a 2 1/2-year FBI investigation of contracts awarded by the lottery board. U.S. Attorney Joseph E. diGenova has said further indictments are expected.

Mason worked for nearly five years as an assistant director of the D.C. Office of Business and Economic Development, where he provided financial advice to minority-owned firms. He was being paid $45,000 a year when he resigned in June 1983.

Prosecutors said that on Jan. 20, 1984, the day before the lottery board approved two contracts for Cho worth nearly $50,000, Cho gave the man identified by sources as Mason a $5,000 check intended for Exum.

Balland said the $5,000 was a payment to Mason for his consulting work, not a payoff.

Assistant U.S. Attorney David Eisenberg said in court that Exum received only $500 from the man. Balland said that the $500 was a loan from Mason to Exum, which has not been repaid.

Eisenberg said that when Cho learned from Exum that he did not receive the full $5,000, Cho subsequently made three cash payments to Exum totaling $5,000.

Cho, who no longer has any contracts with the lottery board, received immunity from prosecution for his cooperation. His lottery contracts were for storing printed lottery tickets and leasing space to the lottery board.

Exum, who could receive up to 12 years in prison and a $112,000 fine when he is sentenced May 30, pleaded guilty to conspiring to accept an illegal gratuity, tax evasion and receiving unemployment benefits while employed by the city.

In an unrelated case, Mason is facing a felony theft charge in D.C. Superior Court in connection with his obtaining a $12,800 D.C. government check issued to a firm that he was working for, according to court records.

Balland said that the charge resulted from a business dispute Mason had with the firm, and that Mason had no criminal intent when he obtained the funds. Balland said that Mason hopes to settle the matter and have the charges dropped.