By the time Alice H. Riley nervously told the court last September how she had altered records and backdated letters for the boss, 22 days had slid by in the trial of the United States v. Marvin Mandel et al.

Mrs. Riley was still nervous when she testified again on Friday, but in this fast-moving second trial on Governor Mandel she took the stand just 16 days after U.S. District Judge Robert L. Taylor began the proceedings.

If this pace keeps up - which it undoubtably will under Taylor's stewardship - the case will go to the jury about one month earlier than it would have at the last trial, which was aborted in December.

It will be relief to everyone - defendants, attorneys and press alike - when the jury finally returns a verdict sometime in August on Mandel and his five codefendants W. Dale Hess, Harry W. Rodgers III, William A. Rodgers, Irvin Kovens and Ernest M. Cory, Jr.

The grand jury investigation of this, that latest chapter of a six-year U.S. investigation of Maryland's political system, began in spring of 1974. The indictments were handed down in November of 1975, a multicount indictment charging Mandel with accepting gifts - now totalling some $350,000 - from his five-codefendants in exchange for his manipulation of the governor's office to benefit his co-defendant's business. It is an alleged scheme of hidden favors and bribed on both sides that the federal prosecutors say was a fraud aginst the citizens of Maryland.

Few cases have had as much difficulty in reaching a final verdict as this one. After three months of testimony last fall a mistral was declared because of two outside attempts to tamper with the verdict. The trial was re-scheduled for April 13, a new presiding judge chosen, and then there was two more postponements because Mandel suffered what doctors believe was a small mild stroke and was hospitalized for 22 days.

Now Taylor is urging the attorneys on both sides to "get on" with the trial and they have compiled. Already testimony is running three trial days ahead of the last case. Jury selection was four calendar days shorter even though this jury is sequestered and therefore its members more difficult to select.

With his stubborn Tennessee charm and his uninhibited lectures to the attorneys, Taylor has prodded defense and prosecution to stipulate to the testimony of 11 witnesses. That means the seven sets of attorneys were able to agree on the essence of the witnesses' testimony (which is possible because they heard it at the last trial and then a summary of the testimony was read to the jury rather than calling the witnesses to the stand. The stipulations have saved hours.

Taylor has also banned bench conferences and chambers conferences during the open court sessions. He has taken what is euphemistically called an abbreviated schedule, set in consideration of Mandel's health prob- lems, and crammed in more testimony than was fitted in to the full schedule of the last trial.

Taylor says he has taken these steps to insure that the 12 jurors and six alternates are not sequestered needlessly for the entire summer.

If nothing else, attorneys have said this speed means that the families of everyone directly involved in the case will be able to take their long-planned vacations.

It also means that the case will be compressed for the jury now impaneled, a fact that both defense and prosecution think is to their benefit.

Already, the prosecution is about half way through the first phase of its presentation, which deasl with alleged secret gifts to Mandel. After this evidence has been presented, the prosecutors will try to show how Mandel influenced legislation in exchange for the gifts in order to bring profits to the business of his friends and co-defendants.

Mrs. Riley was called to the stand to testify about the hidden business deal between her boss, millionaire co-defendant Hess, and Gov. Mandel. The evidence and testimony produced so far shows that Hess gave Mandel secret a 4/9 interest in Hess' partnership in Security Investment Inc., a lucrative company that leases buildings to the federal government.

Mandel received at least $15,000 in direct payments from Hess for his secret partnership that Hess subsequently tried to cover up, testimony shows. Mandel paid $40 for the incrative partnership interest, according to testimony.

Mrs. Riley testified that she altered ledgers and check stubs. As a result, the checks that were payment for the interest in Security Investment appear as payment for old legal fees that Hess owed Mandel, who is an attorney, according to her back-dated a letter by four years so Hess could produce a document to prove he owed Mandel $15,000 for legal fees.

This is the second gift - or bribe, as the prosecution claims - grought to the attention of the jury in the trial. The first dealt with a cut for Mandel in a real estate venture on the Eastern Shore.

One of the partners in the purchase of the 200-acre farm called Ray's Point was Pimlico owner Nathan Cohen who testified that Hess told him: "We take care of the Governor" by giving him shares in business ventures and then covering it up.

Mandel's share in the farm was estimated at $45,000, although his attorney claimed that the governor lost money on the venture.

Much of the testimony - which began June 13 - has centered on a dizzying world of paper: ledgers, accountants' sheets, check stubs, deeds, partnership papers, incom tax returns, "K-1" partnership tax return forms and the like.

Attorneys have painstakingly led the jurors through the maze of papers that can obscure thousands of dollars. A few jurors have dozed off, overwhelmed perhaps by the details.