An extension that is not 'signed' will probably still work.
Reset your Extension Manager's Loading Policy to 'Unrestricted' and restart SketchUp...
Many older extensions have not been updated as SketchUp versions evolved - however many older ones still work...
Provided that you only install RBZs from reputable sites like SketchUcation's PluginStore, EWH or respected authors' own sites there is no problem in loading older unsigned or sign-expired plugins...
Its usage notes in its 'more-info' link explain how it works... context-menu run NOT via the main menus or by toolbar...
@unknownuser said:
Usage:
Select solid
Right-Click Context-Menu. If the solid is broken the option 'FIX SOLID' is present. If the option isn't present there are no problems with the solid.
Select 'Fix Solid'
You can set two options:
Risky fix: if you choose a risky fix it's possible that a part of the solid will be deleted. This action can be undone with the normal SU undo function. The risky fix try to fix all problems, the normal fix only fixes problems with no risk.
Fix coplanar faces: If you choose no, the edges between coplanar will not be removed.
Please note that there several newer and improved tools like this [it is over a decade old !].
Consider looking in the PluginStore for thomthom's SolidInspector² and Anton's ams_Fixit... [which can fix other things too]