Several high-level East European sources, based on direct contacts with Moscow, reveal that:
* Mikhail Gorbachev, the ''young hopeful'' for the Soviet leadership after the death of Yuri Andropov earlier this month, did not lose out in a power struggle. He is already seen within the leadership itself as very much the man of the future.
* Konstantin Chernenko and three senior, equally veteran colleagues beneath him have themselves acknowledged that Chernenko's leadership is a transitional one. The Politburo, the Soviet Union's policy-making body, made a collective decision in favor of transitional leadership after it had thoroughly and outspokenly thrashed out the future.
* There is and will be no change in the collective-leadership concept. Continuity - that is, allowing Mr. Andropov's domestic policies to continue intact - remains the order of the day under the new leadership.
This inside view of events before and following Andropov's death comes from highly authoritative East European sources, based on talks that some East-bloc leaders had with Mr. Chernenko and some of his associates during their visit to Moscow for the funeral.
American Vice-President George Bush and British Prime Minister Margaret Thatcher have not disclosed any details publicly, but they may have drawn some similar impressions during their shorter encounters with the new Soviet leader.
The choice of Chernenko was determined well before Andropov's passing. It was quite clear to Andropov's Politburo colleagues long before he died that he probably did not have long to live - or that, at best, he could not continue to carry the burden of office.
The end, when it came, however, was ''somewhat sudden.''
One East European leader has since told a senior diplomatic visitor of his concern that so many Western observers had regarded the Chernenko choice as a ''step backward,'' i.e., to a more conservative approach.
Ideas of a power struggle behind the scenes are dismissed by sources who are well-placed enough to have learned what went on inside the Kremlin during Andropov's protracted illness.
''The end came more suddenly than had been expected,'' one such source said. This, in fact, would seem confirmed by Defense Minister Dmitri Ustinov's last-minute cancellation of a visit to India.
''But the new structure of the leadership had been set up well in advance.
''Moreover, men like (Premier Nikolai) Tikhonov (who is 79 years old), Ustinov (75), (Foreign Minister Andrei) Gromyko (74), and Chernenko himself (73 this year) know fully well themselves that they can constitute no more than a transitional leadership.
''There are no essential differences between them, and they know they have to build up some of the younger men - and that, after themselves, one of these will take over.''
In such informed appraisals, Gorbachev, at 52 the youngest man in the Politburo, is already firmly fixed as the man most likely to follow Chernenko.
Already Gorbachev is seen as, in effect, a No. 2 man. Through his Politburo membership, he has achieved seniority remarkably early, by Soviet standards. This confers on him immense credibility for the future.
But Hungarian party leaders are, after a momentary uncertainty, now assured that they can continue on their path of economic reform. Hungarian reform obtained a qualified blessing late in Leonid Brezhnev's leadership. It was still more openly approved of by Andropov.
The Hungarians believe Andropov's ideas about strengthening Communist-bloc economies through the reform process - including within the Soviet Union itself - will not only be continued but will also be intensified.
In this connection, they point to two comments by Chernenko. One was last summer when, in a major party report, he said: ''The struggle to raise efficiency of production and the quality of output is a key element of our work.''
The other comment - still more significant now that he is in charge - was in Chernenko's maiden speech to the Central Committee as general secretary. He said: ''The system of economic management, the whole of our economic machinery, needs a serious restructuring. Work in this direction has only been started.''
To Budapest's pioneers of wide-ranging reform of a communist economy, ''restructuring'' is the key word.
''We believe,'' said one, ''that the stress on the need for economic reform and the will to undertake it will be even stronger now than before.''
In his first reaction to the Chernenko appointment, Poland's Gen. Wojciech Jaruzelski implied similar confidence that Moscow will continue to support his own style of moderate pragmatism. Jaruzelski also is committed to economic reform, albeit less far-reaching than Hungary's because of his country's acute difficulties.
The West must now be patient, believe East European leaders most concerned by the collapse of detente and the standstill in East-West relations.
''There will be no departure from the line and the stimulus Andropov had begun to apply, no changes as such,'' a source said. ''But, inevitably a new leadership has ideas of its own. It seeks and creates a new image - its own personal style or touch - and this in Moscow will take time to grow and show itself.''
The West also is seen as having its own present preoccupations. These include their own waiting for this more distinct profile from the Kremlin to appear, and the approaching American election.
For these reasons, said one source, ''We should not expect any meaningful improvement in the international climate for at least 10 months.''