The government of Iran yesterday paid $419.5 million it owes the Export-Import Bank of the United States, the fifth settlement the fundamentalist Islamic government of Ayatollah Ruhollah Khomeini has made with American lenders in the past six weeks.

The payment arrived here yesterday after 2 1/2 years of negotiations at the London branch of Iran's Bank Melli. It is by far the largest debt settlement of the 20 that Iran has made to American banks under the January 1981 agreement for the release of 52 American hostages held in Tehran for 444 days.

It brings the total amount repaid so far to $895.9 million, about two-thirds of the $1.418 billion set aside in a Bank of England escrow account to cover claims of U.S. banks for loans to the pre-revolutionary government of Shah Mohammad Reza Pahlavi. Since July 5, $660 million has been settled, and Treasury officials expect more soon.

There is no indication, however, that the settlements portend any quick resumption of normal U.S. diplomatic relations with Iran; neither do they mean that the Ex-Im Bank will begin making or guaranteeing loans for sales to that country, government officials said.

Private sources close to the tangled political situation in Tehran said the recent spate of settlements represents the ascendency three months ago of a political faction in the country's revolutionary government that believes Iran needs to put its economic house in order.

This faction was described by a source in close touch with Tehran as "interested in fostering settlements" and as being politically close to Iranian Prime Minister Mir Hossein Musavi.

The faction was reported to have replaced one, at least temporarily, that wants the settlements to fail in the belief that such failure would give them greater power within the country.

"This puts us a long way to resolving all of the banking issues with Iran," said Assistant Treasury Secretary John M. Walker Jr., who announced the claims settlement.

"It indicates a desire on the part of the Bank Markazi Iran's central bank to pay off its debts, normalize banking relations internationally and restore its credit internationally," he continued.

"The Iranians are doing all they can to restore their credibility in the world banking community," where it had been badly damaged by the hostage crisis, Walker added.

There is some speculation that Tehran is trying to increase its commercial dealings with the West and Japan. U.S. exports to Iran in the first half of this year totaled $97 million, compared with $122 million for all of 1982, and Iran has indicated it wants major construction projects stalled by the 1979 revolution resumed by Japan, whose foreign minister, Shintaro Abe, visited Tehran earlier this month.

The Ex-Im settlement involves $394.6 million in direct loans made by the federally sponsored bank and $24.9 million in loan guarantees--payment of 100 cents on the dollar. Included in the $394.6 million repayment is interest on the money since the Ex-Im Bank declared Iran in default on the loans on Dec. 21, 1979, about a month after Iranian militants seized the U.S. embassy with 66 hostages, 14 of whom were released before the rest.

In return for getting its money, the Ex-Im Bank withdrew claims pending at an Iran-U.S. Claims Tribunal in the Hague, which was set up as part of the hostage release agreement.

There are, however, 37 other guaranteed loans, representing $14 million, not included in the repayments to the Ex-Im Bank because the Iranians said they were transactions to private Iranian companies rather than to the government. The bank has paid off those claims and can seek repayment from Iran through the courts.

The Ex-Im settlement follows by one week a payment of $9.95 million to Marine Midland Bank of New York. Last month, Iran agreed to repay Manufacturers Hanover Trust Co. of New York $136 million, the largest settlement to that time.

Treasury officials said claims remain outstanding from about 20 more banks, although the Iranians may not agree that all of them are valid.