John C. Whitehead is still unknown to most of the American public, but policy-making officials in the State Department and U.S. missions abroad have become well-accustomed to his instructions and decisions since he was sworn in as deputy secretary last July 9.

In that period of 175 days, according to his office, Whitehead has been acting secretary of state for 82 days, nearly half, during the travels or vacation of Secretary of State George P. Shultz.

Key officials said Whitehead is ready and even eager to make decisions, in contrast to his predecessor, Kenneth W. Dam, who was reluctant to act in many circumstances.

"I've been used all my life to making decisions. I don't shy away from them," Whitehead said in a recent interview. The former New York investment banker, who was co-chairman of the firm of Goldman, Sachs & Co. before Shultz recruited him last year, said it was "a little bit daunting" at first to have so much responsibility in so many different unfamiliar foreign policy areas in his first full-time job in Washington.

"I have gotten used to it. I know there is a vast amount of expert talent around this building to help, and I have learned how to draw on it and draw on it quickly.

"As far as making decisions is concerned, it is important that somebody who is in charge be ready to make decisions. Otherwise the whole process stultifies if the person at the top won't decide things.

"The secretary expects me to decide in his absence and does not expect me to defer every decision to him . . . . He expects me to be in charge when he is away," said Whitehead, adding that he stays in touch with Shultz on "important matters."

Whitehead, 63, said he has been "tremendously impressed" by the high quality and ability of the State Department and Foreign Service people.

He is moving toward a tentative judgment, though, that "the organizational structure" of the State Department may need updating.

WHAT'S IN A (FIRST) NAME . . . Japanese newspapers recently reported as "a very important development" in U.S.-Japan relations a private letter in late November from Secretary Shultz to Japanese Foreign Minister Shintaro Abe. The importance was not the subject -- another U.S. appeal for trade concessions -- but the "Dear Shintaro" greeting. The press reports said it was the first time in three years of official association, including more than 20 personal meetings, that Shultz wrote to his Japanese counterpart on a first-name basis rather than "Dear Minister Abe."

"It's about time," a Foreign Ministry official in Tokyo told the Japan Times. The official said Abe quickly sent back a "Dear George" reply to continue the intimacy.

Abe, who is believed to aspire to be prime minister, was doubtless mindful that Prime Minister Yasuhiro Nakasone has made a political asset in Japan of his "Ron-Yasu" relationship with President Reagan.

PERSONNEL NOTES . . . Gaston Sigur, Asian affairs specialist on the National Security Council staff, will be nominated to become assistant secretary of state for East Asian and Pacific affairs. He will succeed Paul D. Wolfowitz, who is to be nominated as ambassador to Indonesia.

Two senior department officials -- Morton I. Abromowitz, director of research and intelligence, and H. Allen Holmes, director of politico-military affairs -- will soon be getting more impressive new titles. Congress, responding to years of urging by the State Department, appears ready to make these two positions assistant secretaryships subject to presidential nomination and Senate confirmation.

The title change will not mean a pay raise or any change in the jobs now performed by Abromowitz and Holmes. Both positions have been regarded unofficially within the department as equivalent to being an assistant secretary.

However, Congress' past reluctance to recognize that fact had put the bureaus of research and intelligence and politico-military affairs on a lower rung of the department's official pecking order. Now, department officials say, the impending change is expected to improve morale within the two bureaus and give their work and views greater authority within the department.