With a score of 8.6 and being from 1969, I tracked down this movie to see for myself if the hype was justified. The copy I obtained was a fairly damaged print in 4:3 format, quite grainy with washed out colors. It was watermarked 'C.S.C. Roma' in the bottom right corner, suggesting a VHS copy drawn from an archived 35mm (16mm?) print. No commercial releases seem to exist. The sound was in artificial stereo that produced a strange echo. It was effective in the opening war sequence, to be honest. The monologue by the protagonist switched between German (he was a Nazi soldier after all) and Italian for no apparent reason.
The film itself is very slow, pensive, unusual for a war movie of the time, but clearly the director wanted this to be an art film (and it was indeed in competition at the Cannes film festival in 1969). This was Andreassi's only feature film and it shows. He directed several documentary shorts before this and made only one more film (a documentary) 31 years after this. So basically his career was over after FLASHBACK.
Another user here has remarked on the rave reviews this film has received and how they seem to be coming from Scandinavian IMDb users, maybe fans of the Norwegian star of the film, Fred Robsahm, who only made a handful of appearances in B-movies in his career, and whose performance here is rather indifferent. Some of these reviews are so over-the-top, it's clear they are fake.
How this obscure and rare movie has garnered so many user ratings (1,075 at the time of writing) is a mystery in itself. Usually movies this difficult to find have only a few dozens of votes. I conclude that somebody managed to hack the IMDb in this respect (puts SHAWSHANK REDEMPTION in another light, doesn't it?).
Bruno Nicolai is falsely credited as the film's composer, he is not listed in the credits. In fact, there is no score whatsoever in this film! I wrote to the IMDb staff so that this misinformation gets corrected.
If you must see FLASHBACK, do so, but you will find it slow and repetitive. The acting is below average. It's not worthless, but no classic, definitely not.