One of the most important economic jobs in any administration is that of undersecretary of Treasury for monetary affairs. He's the man primarily responsible in two distinct areas: management of the federal debt, and policy on international monetary and financial relationships. Overall, it's the key diplomatic job at a working level in international financial matters, including the role of the dollar in foreign markets.

In the Reagan administration, it is one of the hotly sought-after assignments. Those "supply-siders" who were disappointed in getting only one of their group into a Cabinet-rank slot -- David Stockman as director of Office of Management and Budget -- hope Reagan will name Lewis Lehrman to the undersecretary post. Lehrman, New York entrepreneur, believes that reasonable price stability can best be achieved by returning to the gold standard.

But the odds are lengthening against Lehrman's moving into the sensitive Treasury slot, once occupied by worthies such as New York banker Robert V. Roosa, Federal Reserve Board chairman Paul A. Volcker, and the most recent occupant, New York Federal Reserve Bank president Anthony Solomon.

Talk of going back to gold scares the living daylights out of most conservative economists, including some traditional Republicans in the Reagan camp. "Going to a gold standard would be a move of despair," Nixon's economic council chairman, Herbert Stein, told me. Stein says that the situation is not yet that bad.

Lehrman is not a one-issue man. He is known to believe that a coherent economic policy must stress equally four or five major themes, all dovetailing closely with basic Reagan views. But his controversial stand on gold -- a view for which he is best known -- could eventually knock him out of the box.

The Republicans seem acutely aware of the critical nature of the Treasury undersecretary job -- in substance and symbolism -- although President Carter unaccountably has left it vacant since March 1980, when Solomon went to the New York Fed.

Treasury Secretary G. William Miller and Solomon never did hit it off. Solomon (who had been in the job from the start of the Carter administration, under W. Michael Blumenthal) felt that in late 1978 and early 1979, the administration was letting the economy get out of hand, and that Miller himself was a poor secretary.

Treasury insiders now conclude that Miller wanted to show Solomon that the Treasury could do just fine without him and that Deputy Secretary Robert Carswell and Assistant Secretary C. Fred Bergsten between them could share Solomon's responsibilities as well as their own jobs. Miller made only one pass at filling the job: he offered it to Brookings President Bruce MacLaury, who turned it down.

Everything considered, Carswell and Bergsten did well enough. But as a shrewd New York financial man pointed out, "To the foreigners who pay close attention to these things, the senior position wasn't filled. What that says is that the function was downgraded." White House officials, queried this week, say they never sent any names over to Miller, and don't know why Miller didn't come up with any successful candidates of his own.

"Miller is a funny guy," says a Treasury insider. "He's a loner. His inclination is not to go out and get people. In a sense it was a put-down to Tony. And his rear end was covered because both Carswell and Bergsten could do the substance of the work."

Curiously enough, in his two top jobs in Washington -- chairman of the Federal Reserve and secretary of Treasury -- Miller made virtually no appointments of his own. At the Fed, he took over the staff (including the confidential assistant) left by Arthur F. Burns. At Treasury, he kept all of Blumenthal's personal staff, including press aide Joseph Laitin, despite warnings from the White House that Laitin's leaks had caused Blumenthal trouble.

Wall Streeters were not surprised, after mid-year, that the Solomon job continued to be vacant. At that point, it would have been difficult to attract a quality candidate. Now, the Street is keeping its fingers crossed: it would be apprehensive about Lehrman in the undersecretary role, especially because it regards Treasury secretary-designate Donald Regan -- a successful business manager -- as untested in a new and highly complex Washington environment.