The Marble Hornets Tapes are entries which delve around the footage regarding the Marble Hornets film project that Alex had intended to make, as well as behind-the-scenes of Marble Hornets. Any video tapes which occur during the filming of Marble Hornets or when Alex obsessively filmed himself taking place before Entry #52 fall into this grouping. This does not include the remaining entries of the series itself.
Since the tapes lack any discernible time stamp, the timeline of these recordings is difficult other than them occurring before Entry #52 and after Entry #2.
In Entry #69, Jay and Tim find several tapes that are assumed to be Marble Hornets tapes, as they've been burnt and melted (save two), more than likely by Alex, who said that he would burn the tapes. Jay managed to get two of these tapes working again, and uploaded them as Entries 70 and 71