After four months of the largest product recall in the nation's history, the Firestone Tire & Rubber Co. has replaced an estimated 2 million of a possible 13 million defective "500" steel-belted radial tires.

Firestone officials said yesterday that they had sent only 150,000 recall notices to customers to date -- far fewer than had been anticipated by federal safety officials involved in the voluntary recall order.

Angry consumer advocates yesterday accused the company of dragging its feet in the recall which Firestone agreed to after protracted negotiations with the Department of Transportation.

"It's unbelievable what is going on there," said Ralph Nader. "The notification system must be in a shambles. How many tires would they have recalled if they did tell people about it?"

For its part, the company said it was now sending out an estimated 75,000 letters a week, but had been slow in sending out letters earlier because it was waiting to have replacement tires available.

"We are continuing to manufacture 400,000 tires a month to meet the demands of the recall," a Firestone spokesman said yesterday. "As the tires become available, we are sending out notification letters."

The spokesman confirmed that the number of people requesting replacement of their 500's has "slackened off considerably," since the recall first began last November.

"It appears," the spokesman said, "that people are waiting for when they have to take off their snow tires before they get their tires switched."

In an interview with Dow-Jones yesterday, Firestone chairman and chief executive officer Richard A. Riley said the company expects to replace some 6 million tires at no cost to consumers and another 4 million at halfprice.

"The recall prevented us from making some sales, especially of radial tires," Riley said.

In a related matter, documents sent by Firestone to the Commerce, Consumer, and the Monetary Affairs Subcommittee of the House Committee on Government Operations indicate that Firestone had shipped more than 50,000 steel-belted radial 500 tires overseas.

Firestone records sent to subcommittee chairman Benjamin Rosenthal (D-N.Y.) reveal that between 1973 and October, 1977, the company shipped 55,559 of the tires that would qualify under the recall, to 62 countries, not including tires that were already on vehicles shipped overseas.

The shipments range from one tire sent to San Salvador to nearly 40,000 sent to Japan.

The company told Rosenthal that there was also a possibility that independent jobbers or dealers shipped even more of the tires overseas.

It is unclear how the company plans to replace any of the exported tires, because they are unable to determine which tires shipped overseas would have qualified for the recall based upon the manufacturing dates agreed upon with the government.

"It isn't practical to keep records of all tire shipments by individual tire serial number," the company wrote, "thus we are not able to separate exported tires by dates of manufacture to determine which tires may have been covered by the recall."