The government agency head who last month accused Lockheed Aircraft Corp. of overcharging the government by more than $10 million on Navy shipbuilding contracts has retreated from his charges - but not abandoned them - in a carefully worded letter to Lockheed.

Goodwin Chase, chairman of the Renegotiation Board, conceded in his letter to Lockheed that he had made a mistake in part of the computations that led to his charges, and added, "If the figures calculated by your internal auditors are correct, my figures . . . are overstated."

Sources said Chase had erred in using an average stell price of 10 cents per pound to calculate the amount of unaccounted-for steel. The proper figure, other sources said, is about 83 cents per pound.

But nowhere in his Aug. 17 letter to Lockheed Chairman Robert W. Haack did Chase back down from his charge that Lockheed billed the government for steel it did not use in building seven vessels for the Navy.

In June, Chase told the Senate Banking Committee that Lockheed couldn'to account for 73 millions pounds of steel for which it had billed the navy as part of the cost of the docks, which are called LPDs. Later, Chase upped the estimate to 117 million pounds, worth $10.2 million.

Lockheed replied that there was no unaccounted-for steel, and audited inhouse, 28,000 purchase orders to prove it point. Lockheed's regular auditor, Arthur Yount & Co., reviewed the internal audit, but never audited the purchase orders, Lockheed officials said.

After weeks of coast-to-coast verbal sparring, Lockheed officials and Renegotiation Board staffers met here Aug. 5. It was at that meeting that Lockheed gave Chase the information that resulted in his letter to Haack.

All of this happpens at a time when the staturoty life of the Renegotiation Board, which is responsible for recovering excessive profits from government contractors, is about to come to an end.

Pending in both houses of Congress is legislation to extend the board's life through 1962, and to extend its power to recover money from companies which, like Lockheed, produce several different kinds of products for the government.

What is most puzzling about the current dispute between Lockheed and the board is that two simple sets of figures could seemingly resolve it. The questions are how much steel Lockheed billed the Navy for, and how much steel went into the ships.

Chase said the board does not have the figures, because they are Lockheed's. Lockheed says it has the figures, but refuses to release them to the press, and apparently has not gotten them to Chase.

Meanwhile, Chase, in his letter to Haack, raised the possibility of a "relationship" between the LPD contracts and some 1972 contracts for Navy destroyer escorts that the Justice Department is now investigating.

Chase refused to elaborate on his letter's statement that "questions remain" about the two contracts. A Lockheed spokesman said the two matters are entirely separate.