FROM symbols to substance, United States Secretary of State James Baker III's two days in Moscow were filled with reminders that the Soviet Union is gone and that, increasingly, Russia has taken its place.From the opening talks with Russian Foreign Minister Andrei Kozyrev on Dec. 15 to the four-hour meeting Dec. 16 with Russian President Boris Yeltsin in the glittering St. Catherine's Hall in the Kremlin, once reserved solely for Soviet leaders, the Russian government has repeatedly asserted its authority as the de facto replacement for the Soviet state. Mr. Yeltsin is always careful to refer to the newly formed commonwealth as the new source of legitimacy supplanting the former Soviet Union. "We will, as equal partners, develop the Commonwealth of Independent States," he declared in a press conference after the Kremlin session. The original three founders - Russia, the Ukraine, and Byelorussia - will be joined by six more at a meeting Dec. 21. Commonwealth members will conclude a collective security treaty that will put control of nuclear weapons in the hands of a single command but with political decisions made only through agreement among commonwealth leaders, he said. But the Russian leader also said enough to send another message - that Russia sees itself as the inheritor of much of the Soviet state, in symbols as well as its huge and valuable machinery. The presence of Soviet Defense Minister Marshal Yevgeny Shaposhnikov and Soviet Interior Minister Viktor Barannikov at Yeltsin's side was calculated to communicate that Russia, not the lingering apparatus of Soviet President Mikhail Gorbachev, now commands their millions of troops. "While the collective security treaty has not been signed yet, who is the military to lean upon now but the leadership of Russia?" says Maj. Gen. Geliy Batenin, military adviser to the Russian Ministry of Foreign Affairs. Mr. Baker's meeting with Mr. Gorbachev later Dec. 16 seemed more a gesture of respect than a working session. The Soviet president, who has resisted the new commonwealth, acknowledged that the country "is in transition to a different authority, to a different state." Baker paid homage to Gorbachev's historical role and his "political courage in beginning these transformations and carrying them through." The US is already moving rapidly to accommodate itself to the new political realities here. The US was prepared to discuss diplomatic recognition with the Ukraine, whose leaders Baker was to meet Dec. 18, along with a stop in Byelorussia. Kazakhstan, where Baker stopped Dec. 17 in a swing that included the Central Asian republic of Kirghizia, declared its independence Dec. 16. But Baker admitted to being surprised by the Russian demand for recognition which he said he "heard for the first time" Dec. 15. Russia has never formally declared independence, in part because it seeks to retain the status of the former Union. Russia asks not only for recognition as a separate, independent state, Yeltsin said, but also the "possible succession of Russia to the seat of ... the former Soviet Union in the [United Nations] Security Council that is becoming vacant." In almost passing references, Yeltsin also laid claim to other assets of the Soviet state. The Interior Ministry will not exist on a commonwealth level, he explained, "so, the existing ministry will simply become a part of the Ministry of the Interior of the Russian Federation." On the same day, the Russian parliament placed all the assets of the Soviet parliament - its buildings and bank accounts - under its control. And Russian television news reported that central television and radio stations were be ing taken over as well. The most sensitive issue is the control of the vast Soviet military, almost 4 million strong, armed with some 30,000 nuclear weapons, and long the only true manifestation of this country's claim to be a superpower. The commonwealth agreement compels Russia to share this awesome attribute of statehood, but the specifics of the division are far from clear. Baker's principal concern is the safety and control of the nuclear weapons based in Russia, the Ukraine, Byelorussia, and Kazakhstan, a subject he said was discussed in "quite a bit of specific detail" in the meeting with Yeltsin. The commonwealth agreement provides for a unified command of the country's strategic forces, which according to Yeltsin and others in the Russian government includes a broad range of conventional forces as well. On Dec. 17, Yeltsin defined these as "the air force, the naval force, the air defense force, the nuclear weapons - both strategic and tactical - as well as the intelligence gathering operations." A commander in chief will be appointed who will not be able to use these forces without the decision of the heads of the four nuclear-capable republics. In practice, this does not mean four "buttons" on the nuclear trigger, says General Batenin. "There will be one huge button with everyone holding part of it through a system of codes." But he admits they have yet to decide whether any one of the four may have what amounts to a veto or can simply opt out of a decision to use the forces. To further complicate matters, Byelorussia and the Ukraine have stated their desire to become non-nuclear states (Kazakhstan is expected to follow suit), but Russia will remain a nuclear weapons state "for the time being," says Yeltsin. The weapons based on the other three are to be destroyed on their territory, but Batenin says only Russia has the facilities to store and destroy nuclear warheads. "Politically it is a problem," Batenin admits, "especially taking into account that there will be certain difficulties in the nuclear disarmament process and that Russia will emerge as the only nuclear weapons state on the territory of the former Soviet Union. If Russia does not declare its own policy of nuclear disarmament at the highest level - and such a policy is being developed now - that will just give rise to certain apprehensions about what Russia will do with its nuclear potential."