Differences surfaced yesterday in the American and Japanese versions of what market-opening measures President Reagan and Prime Minister Yasuhiro Nakasone had agreed to last week as U.S. trade specialists began organizing for high-level follow-up talks.

U.S. trade specialists said reports from Tokyo indicated the Japanese were trying to narrow the areas of intensive follow-up, which Reagan and Nakasone said are to be supervised by Secretary of State George P. Shultz and Foreign Minister Shintaro Abe.

"We have no misunderstanding about the areas that Secretary Shultz and President Reagan mentioned" in their talks with Nakasone and Abe, said a high-level Japanese diplomat present in Los Angeles. But he declined to specify areas that will be the subject of intense talks on market-opening moves.

"We may spend a lot of time negotiating what was agreed to at the meeting," said one U.S. official.

A senior administration official told reporters in Los Angeles after the Jan. 2 Reagan-Nakasone meeting that the two leaders had agreed to intensive negotiations to open Japanese markets in four areas where U.S. manufacturers are considered highly competitive, but are being blocked from Japan by high tariffs or other restrictions. The sectors are high technology telecommunications devices, forest products, computer electronics and pharmaceuticals and sophisticated medical equipment.

Nakasone took the initiative by bringing up telecommunications, a thorny issue in the United States, and Reagan brought up the other three sectors, the senior administration official said.

The Reagan administration appears to have decided to ignore any differences between the U.S. and Japanese interpretations and to proceed with the follow-up as if there is full accord on what the two leaders agreed to.

Deputy U.S. Trade Representative Michael B. Smith was named to head the group organizing the follow-up for the U.S. side, which will hold its first meeting this morning. The follow-up will be carried out at a high level as an indication of the seriousness with which the administration regards the trade issues.

Although trade talk dominated the Reagan-Nakasone meeting, no high-level American trade officials -- including U.S. trade representative William E. Brock, Commerce Secretary Malcolm Baldrige or any of their principal deputies -- were at Los Angeles.

One trade official said they had been told by the White House that trade would not be a focal point of the Reagan-Nakasone talks, despite an estimated $35 billion Japanese trade surplus for 1984.

Top Japanese officials also expressed surprise that trade played such a major role in the summit talks. One official traveling with the Nakasone party said the night before the meeting that defense would play a larger role than trade, especially with U.S. Defense Secretary Caspar W. Weinberger present at the meeting.

"Nakasone's people did not want to talk trade, but Nakasone did," said an American trade official.

American trade specialists, uninvited to the Reagan-Nakasone talks, now are catching up on what happened in Los Angeles. Treasury Secretary Donald T. Regan and White House National Security Advisor Robert C. McFarlane briefed other cabinet members Friday on the talks.