Files created in MDTools 975 are not directly compatible with the previous versions of the MDTools 900 series and vice versa.
This issue can be avoided by exporting manifold designs to the MBXML format then importing them into the previous version.
Using the two commands, Export Manifold and Import Manifold, designs from the previous versions of MDTools, such as 970, can be exported to the MBXML format. This can then be imported into MDTools 975.
The same is applicable for files created in MDTools 975 exported to MBXML format and imported into previous versions of MDTools 900 series.
Once imported, designs will be in Virtual design mode but can be converted if designing in Feature mode is preferred.