In a letter to a Senate committee endorsing Bert Lance last January, the then-acting comptroller of the currency failed to mention that Lance and his wife had overdrawn personal accounts at a Calhoun, Ga., bank by well over $100,000, in an apparent violation of banking regulations.

Acting Comptroller Robert Bloom left out that information even though it was common knowledge within the comptroller's office at the time. The comptroller had formally "put an agreement on" the Calhoun bank in the jargon of bank examiners, in December, 1975, forbidding any further overdrafts by the Lances.

Congressional sources revealed yesterday that the Jan. 18 letter from Bloom to Sen Abraham Ribicoff (D-Conn.), chairman of the Governmental Affairs Committee, is now a subject of an investigation into the comptroller's office being conducted by the Internal Revenue Service.

That investigation - due to be completed by Sept. 7 - is meant to determine whether officials of the comptroller's office correctly handled matters involving Lance and the Calhoun bank in the past.

Read in light of information in the new comptroller's report on Lance's affairs, made public last week, that original letter provided the Senate with a sketchy account of the comptroller's past dealings with Lance.

Bloom referred only to allegations against Lance that had appeared in the press prior to Jan. 18, and thus did not mention the formal agreement between bank examiners and the Calhoun bank which forbade overdrafts to the Lances.

Bloom wrote Ribicoff:

"It has . . . been correctly reported in the press that the Calhoun bank allowed directors related to Mrs. Lance to overdraw their accounts for varying lengths of time in violation of good banking practice."

Not reported in the press - or mentioned in the letter - was the fact that Bert Lance was one of those relatives, and that Mrs. Lance was allowed overdrafts of more than $100,000.

Bloom's letter continued:

"However, in reponse to criticism of this practice by our examiners, the amounts of all overdrafts were paid at standard rates of interest and the bank suffered no losses in connection therewith."

But according to the comptroller's report issued last week, Lance and his wife paid no interest on overdrafts incurred before June, 1974 - and these were substantial. The report indicates that the bank examiners' later criticism of those overdrafts did not provoke any retroactive interest payments by the Lances.

Ribicoff and his Senate committee colleagues put great weight on acting comptroller Bloom's letter when they voted to recommend Lance's confirmation in January. Bloom told the committee that Lance was "well qualified to serve as the director of the Office of Management and Budget."

During confirmation hearings, Sen. Jacob K. Javits (R-N.Y.) said, "I consider that letter a very important element in this hearing."

Now, committee sources said yesterday, it appears that the letter disguised the true dimensions of the difficulties Lance had previously faced with federal bank examiners. Ribicoff has scheduled new hearings on the Lance affair for Sept. 7.

Bloom's original letter seems to contradict the new comptroller's report on Lance on another point regarding overdrafts of accounts of the Lance-for-Governor campaign. Referring again to press reports. Bloom confirmed that the comptroller had investigated these overdrafts.

A "full investigation," Bloom wrote, included that these overdrafts did not violate the federal law against bank contributions to political campaigns. However, Bloom's letter continued, the matter was referred to the Justice Department because "such determination" of no illegal behavior can only be made by Justice.

The new comptroller's report put that sequence of events in a different light. It said: "The comptroller's office concluded that both administration proceeding [against the Calhoun bank] and a criminal referral were appropriate." The comptroller made the bank promise not to extend any more money to any Lance campaign without prior permission in writing, and referred the record of its inquiry to the Justice Department, which later decide that no prosecution was warranted.

A source in the comptroller's office told The Washington Post two weeks ago: "We had done a good investigation [of this matter] at Calhoun. We were proud of it. But something went wrong after we gave our findings to Justice for prosecution."

The IRS investigation may explain what the bank examiners who studied this matter originally concluded.

Bloom, who returned to his original post as first deputy comptroller of the currency after the appointment of a new comptroller, John G. Heimann, is on vacation and could not be reached for comment.

Bloom, a registered Democrat, had been acting comptroller in the Carter administration. However, Treasury Secretary W. Michael Blumenthal chose Heimann instead.