The Reagan Administration, partially because of congressional pressure, has reversed itself in recent months on two defense decisions that might have violated provisions of arms control agreements with the Soviet Union.

Fifty Minuteman III intercontinental missiles will not be deployed as originally planned because their multiple nuclear warheads might have violated strategic arms limitation talks (SALT) II limits. And identifying devices will be put on American B52 bombers carrying nuclear-warhead cruise missiles so that they can be counted by the Soviets, as required in the SALT II agreement.

In addition, late last month with almost no publicity, representatives of the United States and Soviet Union began the required five-year review of the 1972 anti-ballistic missile (ABM) treaty in Geneva with the Reagan administration saying it was satisfied with compliance to date and would not ask for changes, according to Capitol Hill sources.

These moves come at a time when the Reagan administration is being questioned by American critics and the Soviets about its argument that "Dense Pack" deployment of the new MX intercontinental missile would not violate either the SALT I or SALT II arms control agreements.

An expert on the Soviet military who formerly worked in the Pentagon said earlier this week that the administration has been sending mixed signals to Moscow on its arms control policies. But he added that it was not by design. "There is no one coordinating these steps," he said.

In the case of the Minuteman III missiles, congressional pressure played a large role in the administration's change of direction. Last January, the Air Force budget contained a request for $15 million to place 50 Minuteman III missiles, each with three warheads, in silos now occupied by older, single-warhead Minuteman IIs.

Several months later, that proposal was dropped during the House-Senate conference on the fiscal 1983 defense authorization bill. Administration support for the funds was reversed, according to Senate sources, after House members and administration arms control officials raised questions about it.

According to one House staff member who worked on the bill, congressmen objected to the plan because the new missiles would bring the total number of U.S. multiple-warhead weapons over SALT II limits. In addition, the Minuteman II silos, another House aide said, would have to be modified beyond what was allowed by the treaty language.

Instead of being deployed, the 50 Minuteman III missiles were put into storage as part of the replacement stock for the six missiles the Air Force tests each year.

The Air Force also had planned to save $89 million by not putting a SALT II-required identifying device on B52s that carry cruise missiles. It was listed earlier this year among the savings made in the Defense Deparment under Secretary Caspar W. Weinberger and justified by the failure of the Senate to ratify to the SALT II agreement.

Now, however, the modifications, which consist of putting visible metallic holders on the bomber's wing, are being made. The so-called FRODS (for functional related operational differences) are being added to the B52Gs that have been turned into cruise missile carriers so the Soviets can identify and count them. This makes it easier for the Soviets to be certain the United States is complying with SALT II limits on the number of nuclear weapons carrying multiple warheads.

Early in the Reagan administration, other proposed modifications of new nuclear weapons systems were turned down within the Pentagon because they would violate SALT II limitations. One was a plan to increase the cruise missile load of the new B1B bomber, which could take up to 22 of the self-navigating missiles. But it was kept at the SALT II limit of 20.

Another possibility was to increase the number of individual warheads on the proposed MX missile. It could carry up to 14, according to Air Force sources, but will be held to the limit of 10 set by SALT II.

All these Reagan administration actions to keep the defense program in conformity with the existing arms agreements are drawing new criticism from conservative Republicans.

Sens. Jesse Helms (R-N.C.), John East (R-N.C.), and Steven D. Symms (R-Idaho), sent a letter to Reagan in September asking him to postpone the ABM treaty review until after the MX decision was made.

In response to that letter, Kenneth M. Duberstein, the assistant to the president for legislative affairs, wrote Helms on Nov. 10 that it was "not feasible or desirable to delay" the Geneva talks. He also made clear that the review would "not foreclose any options" if changes in the treaty were later required.

The American representative to the Geneva sessions, Gen. Richard Ellis, former head of the Strategic Air Command, reportedly told the Soviets that the United States reserved the right to ask for amendment or renegotiation of the ABM treaty at a future date. He also told the Russians that the Reagan administration was "thus far satisfied with compliance" of the treaty's provisions, according to sources on Capitol Hill.

Administration officials maintained last week that the treaty review did not prevent reopening its provisions at a later date. And one official said yesterday that an inter-agency committee is still reviewing the ABM treaty's provisions based on the needs of the proposed deployment for the MX system.

In briefings on Monday, however, Pentagon officials said any plans for a future ABM system to defend the Dense Pack deployment would be made within the constraints of the existing treaty. That agreement permits each country to have one ABM system with 100 interceptor missiles.