For those, who like conspiracy.

Considering the criminal case.
Almost all people think that the No-number Ivanov's case was the real and the only one "Dyatlov group case." Even Vozrozhdenny at the Sverdlovsk medical lab and specialists at the Sverdlovsk criminal lab believed so. They didn't know about existence of another case, they also had never seen the No-number case.
However, Okishev's recollections and thorough inspection of the No-number case make to believe that there were two parallel investigations. One was the case that was initiated by the Special Prosecutor's Office for Supervision of the Testing Facility, and the other was the case initiated by Tempalov, which was later transferred to Ivanov. When the two investigations crossed, Ivanov was forced to stop his region-level case. But this understanding also might not be correct.
There was only one case. It was to be initiated by the Special Prosecutor's Office for supervision of the Testing Facility (which was responsible for the incident), and very quickly lifted to the Prosecutor's Office of the USSR. The reason for initiating the case was a technological incident during the testing. The case was to be aimed against the designers or production personnel, who were responsible for the incident. Deaths of the hikers were not a trigger for that case at all. It is even possible that no case would have been initiated, if the hikers had died independently from the testing. When it turned out that nine civilians had also died as a result of the incident, that episode was added to reinforce the charges. As a result, the Prosecutor's Office of the Sverdlovsk region was asked to collect materials on that episode by the Prosecutor's Office of the USSR.
After that, everything continued in full compliance with the requirements of the RSFSR Criminal Procedure Code, except that the case was classified. Therefore, not all investigators working for the USSR Prosecutor's Office knew who they were working for or what case they were investigating. Measures were taken to conceal the purpose and direction of the investigation.
Everything that had value for the case was sent to Moscow. But due to the measures taken to disguise the goals and direction of the investigation, a lot of unnecessary "garbage" appeared, which was not allowed to litter the case investigated by the USSR Prosecutor's Office. Therefore, such "garbage" had to be cut off and left in Sverdlovsk. The only possible and reasonable instrument to do that was to create a separate dummy case (the No-number case). That's where the no-value papers were dumped. Apparently, Moscow requested that case, taking from it what still could be used. The remaining final "garbage" was returned to Sverdlovsk for submission to the archive of the Sverdlovsk Regional Prosecutor's Office.
The case that was investigated by the USSR Prosecutor's Office was either dismissed or sent to one of the special courts and ended with a court verdict. In the first case, it is stored in the special archive of the USSR Prosecutor's Office. In the second case, it is stored in the archive of one of the former special courts. In any case, the surveillance proceedings for this case are stored in the "closed" archive of the former USSR Prosecutor's Office. It is highly likely that the number 3/2518-59 refers to the surveillance proceedings.
The materials from the No-number case had been stored in Sverdlovsk for many years. Then, instead of destroying them after the expiration of the storage period, somebody who had power decided to make them public, counting that people would not find anything valuable in these materials. Probably, that was a commercial project, which was successfully implemented.