Around May 2015, Andrea “Mancausoft” Milazzo got in touch with Jakub Filipowicz, a Polish guy involved in MERA-400 computer historical researches; Jakub was writing an emulator of this machine, but the operating system was missing and almost unavailable (details on the mera400.pl website [Polish]).
Jakub found 5 magnetic tapes at the Warsaw Museum of Technology, containing hopefully copies of the CROOK operating system. The Museum was not able to read them. After some months, he managed to get the tapes, to try a data recovery, extracting the operating system.
Fascinating story with tons of details, definitely a must-read. Interestingly enough - or sadly enough - I can't seem to find a whole lot of information on the MERA 400 in English, and since I don't speak or read Polish, I can't really give much more information than you can find in the source article. There is a Wikipedia page on the MERA 400's progenitor, the K-202.