In SnapGene 5.2 major changes were made to the way History information is stored.

The History information in files created, or edited and saved in SnapGene 5.2 or later, or SnapGene Viewer 5.2 or later, cannot be read by earlier versions of SnapGene.
Files created in SnapGene 5.2 or SnapGene Viewer 5.2 or later should not be used in SnapGene 5.1 or earlier as History information will not be read or retained if the file is saved with the older version.