The Federal Reserve Board took final steps yesterday to pave the way for extension of its "discount window" to thousands of non-member banks, thrift institutions, credit unions and mutual savings banks beginning Sept. 1.

The board issued revised regulations liberalizing both the proposed lending restrictions -- and new reserve requirements for these institutions -- that it had published last June. The earlier proposals had drawn industry opposition.

The action clears the way for these institutions to obtain access to the Fed's discount window for the first time. The step is required by new legislation. Before, the window was open only to the Fed's member banks.

The board also reserved the right someday to revive the surcharge it imposed in March on the interest it charges at the discount window in certain circumstances, but it didn't say specifically what cases that might involve.

The Fed imposed a "surcharge" -- or higher interest rate -- on discount window loans as part of President Carter's March 14 credit controls. However, that only affected a few banks that were labeled as "frequent borrowers."

The discount window is the phrase used to describe the loans that the Fed make to certain financial institutions to keep the banking system running smoothly. Most of these involve overnight or seasonal loans.

In previous years, the discount window has been available only to banks that were members of the Federal Reserve System. However, the Monetary Control Act passed earlier this year extended it to other institutions as well.

The changes the Fed made from its previously proposed regulations primarily involve rules and reserve requirements that would affect smaller banks and thrift institutions.

Previously, the board had proposed forcing all institutions that use the discount window to keep maintain their reserves at required levels on a weekly basis and to report to the Fed each week on their financial positions.

The revised regulations, however, would forestall any reserve requirements until May for institutions with deposits of less than $1 million, with the possibility that they might be exempt even beyond then.

They also would ease the previously proposed reporting requirements to allow institutions with deposits of $1 million to $5 million to report only once each quarter, rather than weekly.

In addition, the board agreed to allow institutions such as savings and loans to borrow from the Fed's discount window even though they have access to funds from "special" lenders, such as the Federal Home Loan Bank Board.

The earlier regulations would have barred them entirely.